Perovskite oxide, process for producing the perovskite oxide, piezoelectric body, piezoelectric device, and liquid discharge device

ABSTRACT

A process for producing a piezoelectric oxide having a composition (A, B, C) (D, E, F)O 3 , where each of A, B, C, D, E, and F represents one or more metal elements. The composition is determined so as to satisfy the conditions (1), (2), (3), and (4),
 
0.98≦TF(P)≦1.01,  (1)
 
TF(ADO 3 )&gt;1.0,  (2)
 
TF(BEO 3 )&lt;1.0, and  (3)
 
TF(BEO 3 )&lt;TF(CFO 3 )&lt;TF(ADO 3 ),  (4)
 
where TF(P) is the tolerance factor of the perovskite oxide, and TF(ADO 3 ), TF(BEO 3 ), and TF(CFO 3 ) are respectively the tolerance factors of the compounds ADO 3 , BEO 3 , and CFO 3 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a perovskite oxide, a process forproducing the perovskite oxide, a ferroelectric compound containing theperovskite oxide, a piezoelectric body, a piezoelectric device using thepiezoelectric body, and a liquid discharge device using thepiezoelectric body.

2. Description of the Related Art

Currently, piezoelectric devices constituted by a piezoelectric body andelectrodes are used as, for example, actuators installed in inkjetrecording heads. In the piezoelectric devices, the piezoelectric bodyexpands and contracts according to increase and decrease in the strengthof an electric field applied from the electrodes to the piezoelectricbody in a predetermined direction. For example, perovskite oxides suchas PZT (lead titanate zirconate) are known as materials suitable for thepiezoelectric body. Such materials are ferroelectric materials whichexhibit spontaneous polarization even when no electric field is applied.The piezoelectric materials are reported to exhibit high piezoelectricperformance at and near the morphotropic phase boundary (MPB).

PZT is a solid solution of PbTiO₃ (PT) and PbZrO₃ (PZ). FIG. 14 is aphase diagram of PZT with respect to the temperature and the molefraction of titanium (i.e., the mole fraction of PbTiO₃) in PZT. Thephase diagram of FIG. 14 is cited from “Landolt-Bornstein: NumericalData and Functional Relationships in Science and Technology, NewSeries,” Group III: Crystal and Solid State Physics, Vol. 16, edited byK. H. Hellwege and A. M. Hellwege, Springer-Verlag Berlin-Heidelberg-NewYork (1981) p. 426 & FIG. 728. In FIG. 14, FT denotes the tetragonalphase, and FR denotes the rhombohedral phase.

PZT tends to form tetragonal crystals when the Ti composition is high,and rhombohedral crystals when the Zr composition is high. When themolar compositions of Ti and Zr are approximately identical, the MPBcomposition is achieved. For example, the mole ratio of Zr to Ti of52/48, which is near to the MPB composition, is preferable. Thetextbooks on the piezoelectric ceramic materials teach that the crystalstructures become unstable and the piezoelectric performance becomeshighest at and near the MPB. Conventionally, PZT has been reported toform pseudocubic crystals at and near the MPB. However, details of thenanostructure of PZT are unknown.

In the above circumstances, the Japanese Unexamined Patent PublicationNo. 2006-036578 (hereinafter referred to as JP2006-036578) reports thatsintered bodies of PZT-based ceramic materials such as Pb(Ti, Zr, Nb)O₃are formed of two-phase mixed crystals of tetragonal crystals andrhombohedral crystals at and near the MPB. (See, for example, claim 9 inJP2006-036578.) Further, JP2006-036578 discloses that it is possible todesirably design the composition on the basis of the relationshipbetween the piezoelectric coefficient and the phase fractions of thetetragonal and rhombohedral phases. (See, for example, Table 1, FIG. 4,and the paragraph 0027 in JP2006-036578.)

In addition, S. Yokoyama et al., “Compositional Dependence of ElectricalProperties of Highly (100)-/(001)-Oriented Pb(Zr, Ti) O₃ Thick FilmsPrepared on Si Substrates by Metalorganic Chemical Vapor Deposition”,Japanese Journal of Applied Physics, Vol. 42, pp. 5922-5926, 2003 alsoreport that PZT films are formed of two-phase mixed crystals oftetragonal crystals and rhombohedral crystals at and near the MPB. (See,for example, FIG. 2( b) in the Yokoyama reference.)

However, in the conventional piezoelectric devices of a first type, thepiezoelectric effect of expansion of a ferroelectric body in thedirection of the spontaneous polarization is generally utilized byapplying an electric field to the ferroelectric body along the directionof the spontaneous polarization. That is, it has been conventionallyconsidered important to design the piezoelectric materials so that theelectric field is applied along the direction of the spontaneouspolarization. Nevertheless, in the case where only the piezoelectriceffect of expansion in the direction of the spontaneous polarization isutilized, the amount of displacement is limited, although greaterdisplacement is currently demanded.

The Japanese Patent No. 3568107 (hereinafter referred to as JP3568107)proposes the conventional piezoelectric devices of a second type, inwhich application of an electric field induces phase transition in apiezoelectric body. JP3568107 discloses a piezoelectric deviceconstituted by a phase-transition film, electrodes, and a heating body,where the heating body adjusts the temperature of the phase-transitionfilm to a level near to the Curie point Tc. (See claim 1 in JP3568107.)JP3568107 refers to use, as the phase-transition film, of a film inwhich transition occurs between a tetragonal phase and a rhombohedralphase or between a cubic phase and a tetragonal or rhombohedral phase.(See claim 2 in JP3568107.) Further, JP3568107 reports that theconventional piezoelectric devices of the second type disclosed inJP3568107 can achieve greater displacement than the conventionalpiezoelectric devices of the first type because both of thepiezoelectric effect of the ferroelectric material and the change in thecrystal structure associated with the phase transition contribute to thedisplacement.

As explained above, although PZT has been conventionally reported toform pseudocubic crystals at and near the MPB, JP2006-036578 and theYokoyama reference report that PZT-based ceramic materials form atwo-phase mixed-crystal structure containing a tetragonal phase and arhombohedral phase at and near the MPB. However, many aspects of thepiezoelectric mechanism and the crystal structure at and near the MPBare still unknown.

In addition, according to the technique disclosed in JP2006-036578, itis necessary to prepare samples of perovskite oxides each constituted bya plurality of predetermined elements with different mole fractions,obtain the phase fractions of the tetragonal and rhombohedral phases ineach sample by X-ray diffraction and Rietveld analysis, obtain thepiezoelectric coefficient of each sample, and determine the compositionon the basis of the relationship between the obtained phase fractionsand the piezoelectric coefficient. However, according to the abovetechnique, it is necessary to search for desirable composition byperforming an experiment every time the constituent elements of thesample are changed, so that the material design cannot be efficientlymade by the technique.

Incidentally, as mentioned before, JP3568107 refers to the use, as thephase-transition film, of films in which phase transition occurs betweena tetragonal system and a rhombohedral system or between a cubic systemand a tetragonal or rhombohedral system. However, the piezoelectricdevices disclosed in JP3568107 are assumed to be used in the vicinity ofthe Curie point Tc. Since the Curie point Tc corresponds to thephase-transition temperature between the ferroelectric phase and theparaelectric phase, the phase transition between the tetragonal phaseand the rhombohedral phase occurs in no film when the film is used inthe vicinity of the Curie point Tc. That is, the piezoelectric devicesdisclosed in JP3568107 cannot utilize the phase transition other thanthe phase transition between the ferroelectric phase and theparaelectric phase. In addition, since spontaneous polarization does notoccur in the paraelectric material, the piezoelectric devices disclosedin JP3568107 do not exhibit the piezoelectric effect of expansion in thedirection of the polarization in response to application of an electricfield after the phase transition.

The present inventor and colleagues belonging to the present assigneehave proposed in the International Patent Application Publication No.WO2007/034903 (which is hereinafter referred to as WO2007/034903) apiezoelectric device using a piezoelectric body which contains regionsin a first ferroelectric phase having crystal orientation. In thepiezoelectric body, the phase of at least a portion of the above regionstransitions from the first ferroelectric phase corresponding to a firstcrystal system to a second ferroelectric phase corresponding to a secondcrystal system different from the first crystal system when an electricfield is applied to the piezoelectric body.

In the above piezoelectric device, it is possible to achieve a volumechange caused by a change in the crystal structure associated with thephase transition from the first ferroelectric phase. In addition, sincethe piezoelectric effect works in both of the first ferroelectric phase(before the phase transition) and the second ferroelectric phase (afterthe phase transition), the piezoelectric device disclosed inWO2007/034903 exhibits greater displacement than the piezoelectricdevices disclosed in JP3568107 when the electric field is applied to thepiezoelectric body.

Further, WO2007/034903 reports that the engineered-domain effect and thelike increase the distortion amount (displacement) when the directionalong which the electric field is applied to the piezoelectric body isdifferent from the orientation of the spontaneous polarization axis inthe ferroelectric phase before the phase transition, and is preferablyapproximately identical to the orientation of the spontaneouspolarization axis in the ferroelectric phase after the phase transition.

SUMMARY OF THE INVENTION

The present invention has been made in view of such circumstances.

The first object of the present invention is to provide a process forproducing a perovskite oxide on the basis of a novel material designprinciple proposed for designing a perovskite oxide superior in thepiezoelectric performance (i.e., the ferroelectric performance), wherethe process is particularly suitable for producing a perovskite oxidefor use in the piezoelectric device exploiting the phase transitioninduced by application of an electric field as proposed inWO2007/034903.

The second object of the present invention is to provide a perovskiteoxide which is produced by the above process.

The third object of the present invention is to provide a ferroelectriccompound and a piezoelectric body containing a perovskite oxide which isproduced by the above process.

The fourth object of the present invention is to provide a piezoelectricdevice and a liquid discharge device using the above piezoelectric body.

(I) In order to accomplish the above first object, according to thefirst aspect of the present invention, a process for producing aperovskite oxide is provided. In addition, in order to accomplish theabove second object, according to the second aspect of the presentinvention, the perovskite oxide produced by the process according to thefirst aspect of the present invention is provided. The perovskite oxidehas a composition expressed by the compositional formula,(A,B,C)(D,E,F)O₃,  (P)where each of A, B, C, D, E, and F represents one or more metalelements, A, B, and C represent A-site elements, D, E, and F representB-site elements, and O represents the oxygen atom. The A-site elementsA, B, and C may be different, or two or all of the A-site elements A, B,and C may be identical. However, when two or all of the A-site elementsA, B, and C are identical, the B-site elements D, E, and F aredifferent. In addition, the B-site elements D, E, and F may bedifferent, or two or all of the B-site elements D, E, and F may beidentical. However, when two or all of the B-site elements D, E, andFare identical, the A-site elements A, B, and C are different. Althoughthe ratio of each of the total molar amount of the A-site elements andthe total molar amount of the B-site elements to the molar amount ofoxygen atoms is normally 1:3, the ratio of each of the total molaramount of the A-site elements and the total molar amount of the B-siteelements to the molar amount of oxygen atoms may deviate from 1:3 withina range in which the composition expressed by the compositional formula(A, B, C) (D, E, F) O₃ can form a perovskite structure. The processaccording to the first aspect of the present invention is characterizedin that the composition of the compound (A, B, C) (D, E, F)O₃ isdetermined so as to satisfy the conditions expressed by the inequalities(1) to (4).0.98≦TF(P)≦1.01  (1)TF(ADO₃)>1.0  (2)TF(BEO₃)<1.0  (3)TF(BEO₃)<TF(CFO₃)<TF(ADO₃).  (4)In this specification, TF(X) is the tolerance factor of an oxideexpressed by the compositional formula X, although the tolerance factorof the perovskite oxide (A, B, C) (D, E, F)O₃ is expressed as TF(P). Thetolerance factor is defined asTF=(rA+rO)/√2(rB+rO),where rA is the average ionic radius of the A-site element, rB is theaverage ionic radius of the B-site element, and rO is the ionic radiusof the oxygen ion. In this specification, the ionic radius is theShannon ionic radius. (See R. D. Shannon, “Revised effective ionic radiiand symmetric studies of interatomic distances in halides andchalcogenides”, Acta Crystallographica, A32, pp. 751-767, 1976). Theaverage ionic radius is expressed by ΣCiRi, where Ci indicates the molefraction of each ion in a lattice site, and Ri is the ionic radius ofthe ion.

According to the first aspect of the present invention, the tolerancefactor of each of the oxides (A, B, C) (D, E, F)O₃, ADO₃, BEO₃, and CFO₃is theoretically obtained, and the composition of the perovskite oxide(A, B, C) (D, E, F)O₃ is determined as above. At this time, even whenone or more of the oxides do not form a perovskite structure bythemselves, the tolerance factor of each of the oxides is theoreticallyobtained.

Preferably, the perovskite oxide may be produced by the processaccording to the first aspect of the present invention so that theperovskite oxide further has one or any possible combination of thefollowing additional features (i) to (iv), and the perovskite oxideaccording to the second aspect of the present invention may furthercomprise one or any possible combination of the following additionalfeatures (i) to (iv).

(i) It is preferable that the composition of the perovskite oxide (A, B,C) (D, E, F) O₃ be determined so as to further satisfy the conditionexpressed by the inequalities (5). That is, it is preferable that thecomposition of the perovskite oxide according to the second aspect ofthe present invention further satisfy the condition expressed by theinequalities (5).0.98≦TF(CFO₃)≦1.02  (5)

(ii) The phase structure of the perovskite oxide is not specificallylimited. For example, the perovskite oxide may have a three-phasemixed-crystal structure in which the three components ADO₃, BEO₃, andCFO₃ coexist, or a single-phase structure in which the three componentsADO₃, BEO₃, and CFO₃ are completely solid solved into a single phase.Further, the perovskite oxide may have another structure.

(iii) It is preferable that the perovskite oxide according to the secondaspect of the present invention contain the first component ADO₃, thesecond component BEO₃, and the third component CFO₃. In each of thefirst component ADO₃, the second component BEO₃, and the third componentCFO₃, the ratio of each of the molar amount of the A-site element andthe molar amount of the B-site element to the molar amount of oxygenatoms is normally 1:3. However, the ratio of each of the molar amount ofthe A-site element and the molar amount of the B-site element to themolar amount of oxygen atoms may deviate from 1:3 within a range inwhich each of the first component ADO₃, the second component BEO₃, andthe third component CFO₃ can form a perovskite structure.

(iv) In the perovskite oxide having the feature (iii), it is alsopreferable that the first component ADO₃ and the second component BEO₃form structures respectively corresponding to different crystal systems,and it is particularly preferable that the first component ADO₃, thesecond component BEO₃, and the third component CFO₃ form structuresrespectively corresponding to different crystal systems.

For example, the perovskite oxide may have a three-phase mixed-crystalstructure in which the first component forms a first crystal structurecorresponding to one of tetragonal, orthorhombic, monoclinic, trigonal,and rhombohedral systems, the second component forms a second crystalstructure corresponding to one of tetragonal, orthorhombic, andrhombohedral systems which is different from the first crystalstructure, and the third component forms a third crystal structurecorresponding to one of cubic and pseudocubic systems.

An example of the perovskite oxide according to the second aspect of thepresent invention has the composition expressed by the compositionalformula,Pb(Ti, Zr, M)O₃,  (PX)where M in the compound Pb(Ti, Zr, M)O₃ is at least one of the metalelements Sn, Nb, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn, Co, Ni, V, and Fe.

Another example of the perovskite oxide according to the second aspectof the present invention has the composition expressed by thecompositional formula,(Ba, Ca, Sr)(Ti, Zr, M)O₃,  (PY)where M in the compound (Ba, Ca, Sr)(Ti, Zr, M)O₃ is at least one of themetal elements Sn, Nb, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn, Co, Ni, V, andFe.

A further example of the perovskite oxide according to the second aspectof the present invention has the composition expressed by thecompositional formula,Bi(Al,Fe,M)O₃,  (PW)where M in the compound Bi (Al, Fe, M) O₃ is at least one of the metalelements Cr, Mn, Co, Ni, Ga, and Sc.

(II) In addition, in order to accomplish the aforementioned secondobject, a perovskite oxide according to the third aspect of the presentinvention is also provided. The piezoelectric body according to thethird aspect of the present invention is characterized in having acomposition at or near a morphotropic phase boundary (MPB), and amixed-crystal structure composed of at least two first crystal phaseswhich are ones of tetragonal, orthorhombic, and rhombohedral phases, andat least one second crystal phase which is at least one of cubic andpseudocubic phases.

Further, in order to accomplish the aforementioned second object, aperovskite oxide according to the fourth aspect of the present inventionis also provided. The piezoelectric body according to the fourth aspectof the present invention is characterized in having a composition at ornear the morphotropic phase boundary (MPB), and exhibits in ahigh-resolution X ray diffraction profile a first diffraction peak of atetragonal phase, a second diffraction peak of a rhombohedral phase, anda third diffraction peak of a third phase different from the tetragonalphase and rhombohedral phases.

In this specification, the expression “at or near the MPB” means thatthe composition of the perovskite oxide is in a range of the compositionin which the phase of the perovskite oxide transitions when an electricfield is applied to the perovskite oxide.

(III) In order to accomplish the aforementioned third object, aferroelectric compound according to the fifth aspect of the presentinvention is provided. The ferroelectric compound according to the fifthaspect of the present invention is characterized in containing theperovskite oxide according to one of the second to fourth aspects of thepresent invention.

In addition, in order to accomplish the aforementioned third object, apiezoelectric body according to the sixth aspect of the presentinvention is also provided. The piezoelectric body according to thesixth aspect of the present invention is characterized in containing theperovskite oxide according to one of the second to fourth aspects of thepresent invention. The piezoelectric body may be, for example, apiezoelectric film or a sintered body of a piezoelectric ceramicmaterial.

Preferably, the above piezoelectric body according to the sixth aspectof the present invention may further comprise one or any possiblecombination of the following additional features (v) to (viii).

(v) It is preferable that the piezoelectric body according to the sixthaspect of the present invention contain a ferroelectric phase havingcrystal orientation. In this specification, the expression “havingcrystal orientation” means that the degree F. of orientation measured bythe Lotgerling technique is 80% or higher. The degree F. of orientationdefined asF(%)=(P−P0)/(1−P0)×100,where P is the ratio of the total XRD (X-ray diffraction) intensity froman orientation plane to the total XRD intensity from all the crystalplanes, and P0 is the value of P in the case where the sample iscompletely randomly oriented. In the case of the (001) orientation,P=ΣI(001)/ΣI(hkl), where I(hkl) is the XRD intensity from the crystalplane (hkl), ΣI(001) is the total XRD intensity from the crystal plane(001), and ΣI(hkl) is the total XRD intensity from all the crystalplanes (hkl). For example, in the case of the (001) orientation in aperovskite crystal, P=I(001)/{I(001)+I(100)+I(101)+I(110)+I(111)}. Whenthe sample is completely randomly oriented, F=0%. When the sample iscompletely oriented, F=100%.

(vi) It is preferable that the piezoelectric body according to the sixthaspect of the present invention contain a region in a ferroelectricphase (ferroelectric region) having crystal orientation along adirection different from the orientation of the spontaneous polarizationaxis.

(vii) In the case where the piezoelectric body according to the sixthaspect of the present invention has the feature (vi), it is furtherpreferable that the ferroelectric phase be one of a rhombohedral phasehaving crystal orientation along approximately the <100> direction, arhombohedral phase having crystal orientation along approximately the<110> direction, a tetragonal phase having crystal orientation alongapproximately the <110> direction, a tetragonal phase having crystalorientation along approximately the <111> direction, an orthorhombicphase having crystal orientation along approximately the <100>direction, and an orthorhombic phase having crystal orientation alongapproximately the <111> direction. In this specification, the expression“having crystal orientation along approximately the <abc> direction”means that the degree F. of orientation along the <abc> direction is 80%or higher.

(viii) In the case where the piezoelectric body according to the sixthaspect of the present invention has the feature (vi) or (vii), it isfurther preferable that at least a portion of the above ferroelectricphase contained in the piezoelectric body according to the sixth aspectof the present invention transition when an electric field is applied tothe piezoelectric body along a direction different from the orientationof the spontaneous polarization axis.

(IV) In order to accomplish the aforementioned fourth object, apiezoelectric device according to the seventh aspect of the presentinvention is provided. The piezoelectric device according to the seventhaspect of the present invention is characterized in comprising thepiezoelectric body according to the sixth aspect of the presentinvention and electrodes through which an electric field is applied tothe piezoelectric body.

In addition, in order to accomplish the aforementioned fourth object, aliquid discharge device according to the eighth aspect of the presentinvention is also provided. The liquid discharge device according to theeighth aspect of the present invention is characterized in comprisingthe piezoelectric device according to the seventh aspect of the presentinvention, a substrate, and a discharge member. The piezoelectric deviceis arranged above the substrate. The discharge member is formedintegrally with or separately from the substrate, and includes aliquid-reserve chamber and a liquid-discharge outlet, where theliquid-reserve chamber reserves liquid, and the liquid reserved in theliquid-reserve chamber is externally discharged through theliquid-discharge outlet.

(v) The present invention has the following advantages.

-   -   (a) The first aspect of the present invention presents a novel        material design principle for producing a perovskite oxide        superior in the piezoelectric (ferroelectric) performance.        Therefore, according to the first aspect of the present        invention, it is possible to easily design the composition of        the perovskite oxide superior in the piezoelectric        (ferroelectric) performance.    -   (b) The material design principle presented by the first aspect        of the present invention is suitable for designing the        composition of a perovskite oxide for use in the system        utilizing phase transition induced by an electric field as        proposed in WO2007/034903. According to the present invention,        it is possible to provide a perovskite oxide having such a        domain structure that even when the strength of the electric        field applied to the perovskite oxide is relatively low, phase        transition can easily occur and great distortion (displacement)        is produced.    -   (c) It is possible to provide a piezoelectric device having        superior piezoelectric performance by using the perovskite oxide        produced in accordance with the material design principle        presented by the present invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram indicating the tolerance factors TF and the crystalsystems of various perovskite oxides in association with the ionic radiiof the A-site elements and the B-site elements.

FIG. 2 is a diagram schematically indicating a piezoelectriccharacteristic of a piezoelectric body which is entirely composed of afirst ferroelectric phase, and transitions to a second ferroelectricphase when an electric field is applied to the piezoelectric body, wherethe first and the second ferroelectric phases correspond to differentcrystal systems.

FIGS. 3A, 3B, and 3C are diagrams illustrating examples of domains ofthree crystal systems in a phase transition model 1 for explainingeffectiveness of use of the perovskite oxide according to the presentinvention in the system in which phase transition is induced byapplication of an electric field.

FIG. 4A is a diagram schematically indicating the relationship betweenthe potential energy and the distance from the center to the B-siteelement in the unit cell of the rhombohedral crystal.

FIG. 4B is a diagram schematically indicating the relationship betweenthe potential energy and the distance from the center to the B-siteelement in the unit cell of the cubic or pseudocubic crystal.

FIGS. 5A, 5B, and 5C are diagrams illustrating examples of domains ofthree crystal systems in a phase transition model 2 for explainingeffectiveness of use of the perovskite oxide according to the presentinvention in the system in which phase transition is induced byapplication of an electric field.

FIG. 6 is a cross-sectional view schematically illustrating a crosssection of an essential portion of an inkjet recording head (as a liquiddischarge device) having a piezoelectric device according to anembodiment of the present invention.

FIG. 7 is a schematic diagram of an example of an inkjet recordingapparatus having the inkjet recording head of FIG. 6.

FIG. 8 is a top view of a portion of the inkjet recording apparatus ofFIG. 7.

FIG. 9A is a high-resolution XRD (X-ray diffraction) profile of apiezoelectric body in a concrete example 1.

FIG. 9B is a high-resolution XRD profile of a piezoelectric body in acomparison example 1.

FIGS. 10A to 10D are diagrams indicating EXAFS (Extended X-rayAbsorption Fine Structure) spectra of the piezoelectric bodies in theconcrete example 1 and the comparison example 1.

FIG. 11 is a diagram indicating high-resolution XRD profiles of apiezoelectric body in a concrete example 2 which are obtained atdifferent levels of the electric field strength.

FIG. 12 is a diagram indicating a high-resolution XRD profile of apiezoelectric body in a concrete example 3.

FIG. 13A is a diagram indicating an XRD profile of a piezoelectric bodyin a concrete example 4.

FIG. 13B is a diagram indicating a magnification of a portion of the XRDprofile of FIG. 13A around the diffraction angle (2θ) of 46 degrees.

FIG. 14 is a phase diagram of PZT.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is explained in detailbelow with reference to drawings.

1. Perovskite Oxide

As explained in the “SUMMARY OF THE INVENTION”, the first aspect of thepresent invention presents a process for producing a perovskite oxideexpressed by the compositional formula,(A, B, C)(D, E, F)O₃,  (P)where each of A, B, C, D, E, and F represents one or more metalelements, A, B, and C represent A-site elements, D, E, and F representB-site elements, and O represents the oxygen element. The A-siteelements A, B, and C may be different, or two or all of the A-siteelements A, B, and C may be identical. However, when two or all of theA-site elements A, B, and C are identical, the B-site elements D, E, andF are different. In addition, the B-site elements D, E, and F may bedifferent, or two or all of the B-site elements D, E, and F may beidentical. However, when two or all of the B-site elements D, E, andFare identical, the A-site elements A, B, and C are different. Althoughthe ratio of each of the total molar amount of the A-site elements andthe total molar amount of the B-site elements to the molar amount ofoxygen atoms is normally 1:3, the ratio of each of the total molaramount of the A-site elements and the total molar amount of the B-siteelements to the molar amount of oxygen atoms may deviate from 1:3 withina range in which the composition expressed by the compositional formula(A, B, C) (D, E, F)O₃ can form a perovskite structure.

The process for producing the perovskite oxide according to the firstaspect of the present invention is characterized in that the compositionof the perovskite oxide (A, B, C)(D, E, F)O₃ is determined so as tosatisfy the conditions expressed by the inequalities (1) to (4).0.98≦TF(P)≦1.01  (1)TF(ADO₃)>1.0  (2)TF(BEO₃)<1.0  (3)TF(BEO₃)<TF(CFO₃)<TF(ADO₃).  (4)

Preferably, the composition of the perovskite oxide (A, B, C) (D, E,F)O₃ is determined so as to further satisfy the condition expressed bythe inequalities (5).0.98≦TF(CFO₃)≦1.02  (5)In the above inequalities (1) to (5), TF(X) is the tolerance factor ofan oxide expressed by the compositional formula X, although thetolerance factor of the perovskite oxide (A, B, C)(D, E, F)O₃ isexpressed as TF(P).

In addition, as explained in the “SUMMARY OF THE INVENTION”, the secondaspect of the present invention presents the above perovskite oxideproduced by the process according to the first aspect of the presentinvention. That is, the perovskite oxide according to the second aspectof the present invention has the composition expressed by the abovecompositional formula (A, B, C) (D, E, F)O₃, and the perovskite oxideaccording to the second aspect of the present invention is characterizedin that the composition of the perovskite oxide (A, B, C) (D, E, F)O₃satisfies the conditions expressed by the above inequalities (1) to (4).Preferably, the composition of the perovskite oxide (A, B, C) (D, E,F)O₃ satisfies the condition expressed by the above inequalities (5).

In the case where all of the B-site elements D, E, and F are identical,the compositional formula (A, B, C) (D, E, F)O₃ can be expressed as(A, B, C)DO₃,  (P1)where each of A, B, C, and D represents one or more metal elements, A,B, and C represent A-site elements, D represents one or more B-siteelements, O represents the oxygen element, and the A-site elements A, B,and C are different.

In this case, it is preferable that the perovskite oxide (A, B, C)DO₃satisfy the conditions expressed by the following inequalities (1a) to(4a), and it is particularly preferable that the perovskite oxide (A, B,C)DO₃ further satisfy the condition expressed by the followinginequalities (5a).0.98≦TF(P1)≦1.01  (1a)TF(ADO₃)>1.0  (2a)TF(BDO₃)>1.0  (3a)TF(BDO₃)≦TF(CDO₃)≦TF(ADO₃)  (4a)0.98≦TF(CDO₃)≦1.02  (5a)In the inequalities (1a), the tolerance factor of the perovskite oxide(A, B, C)DO₃ is expressed as TF(P1).

Further, in the case where all of the A-site elements A, B, and C areidentical, the compositional formula (A, B, C) (D, E, F)O₃ can beexpressed asA(D, E, F)O₃,  (P2)where each of A, D, E, and F represents one or more metal elements, Arepresents one or more A-site elements, D, E, and F represent B-siteelements, O represents the oxygen element, and the B-site elements D, E,and F are different.

In this case, it is preferable that the perovskite oxide A(D, E, F)O₃satisfy the conditions expressed by the following inequalities (1b) to(4b), and it is particularly preferable that the perovskite oxide A(D,E, F)O₃ further satisfy the condition expressed by the followinginequalities (5 b).0.98≦TF(P2)≦1.01  (1b)TF(ADO₃)>1.0  (2b)TF(AEO₃)<1.0  (3b)TF(AEO₃)<TF(AFO₃)<TF(ADO₃)  (4b)0.98≦TF(AFO₃)≦1.02  (5b)In the inequalities (1b), the tolerance factor of the perovskite oxideA(D, E, F)O₃ is expressed as TF(P2).

FIG. 1 is a diagram indicating the tolerance factors TF and the crystalsystems of various perovskite oxides in association with the averageionic radii of the A-site elements and the B-site elements, where ionsof one or two elements reside in the A sites of the perovskite oxides,and ions of one or two elements reside in the B sites of the perovskiteoxides.

In FIG. 1, C denotes the cubic crystal, M denotes the monocliniccrystal, PC denotes the pseudocubic crystal, R denotes the rhombohedralcrystal, T denotes a tetragonal crystal, and Tr denotes the trigonalcrystal. In addition, the ionic radii of various elements are indicatedin association with the symbols for the respective elements. Inparticular, both of the ionic radius “0.64 (angstroms)” of the trivalentMn ion and the ionic radius “0.67 (angstroms)” of the divalent Mn ionare indicated in FIG. 1.

When the tolerance factor TF is equal to 1.0, the crystal lattice of theperovskite structure has the closest packed structure. Since the B-siteions hardly move in the crystal lattice under this condition, theperovskite oxide is likely to have a stable structure. When theperovskite oxide has such a composition as to realize the abovecondition, the perovskite oxide is likely to have a crystal structuresuch as the cubic or pseudocubic crystal structure, and does not exhibitferroelectricity, or exhibits very low ferroelectricity.

When the tolerance factor TF is greater than 1.0, the B-site ions aresmaller than the A-site ions. Under this condition, the B-site ions arelikely to enter the crystal lattice even when the crystal lattice isdistorted, and to move in the crystal lattice. When the perovskite oxidehas such a composition as to realize the above condition, the perovskiteoxide is likely to have a crystal structure such as the tetragonalcrystal (in which the spontaneous polarization axis is orientated alongthe <001> direction), and exhibits ferroelectricity. There is a tendencythat the ferroelectricity becomes higher when the difference of thetolerance factor from 1.0 increases.

When the tolerance factor TF is smaller than 1.0, the B-site ions aregreater than the A-site ions. Under this condition, the B-site ions donot enter the crystal lattice unless the crystal lattice is distorted.When the perovskite oxide has such a composition as to realize the abovecondition, the perovskite oxide is likely to have a crystal structuresuch as the orthorhombic crystal (in which the spontaneous polarizationaxis is orientated along the <110> direction) or the rhombohedralcrystal (in which the spontaneous polarization axis is orientated alongthe <111> direction), and exhibits ferroelectricity. There is a tendencythat the ferroelectricity becomes higher when the difference of thetolerance factor from 1.0 increases.

Table 1 shows first and second components constituting each of variousmixed crystals and the mole fractions of the first and second componentswhich realize an morphotropic phase boundary (MPB) in each mixedcrystal, where the tolerance factor TF of the first component is greaterthan one, and the tolerance factor TF of the second component is smallerthan one. Table 1 also shows the average ionic radii of the A-site andB-site ions in each mixed crystal, the tolerance factor TF of each mixedcrystal, the crystal system of the monocrystal formed by each of thefirst and second components, the ionic radii of the A-site and B-siteions in the monocrystal, and the tolerance factor TF of the monocrystal.In Table 1, the tetragonal crystal, the orthorhombic crystal, and therhombohedral crystal are respectively indicated by T, O, and R.

As understood from Table 1, the tolerance factors TF of the MPBcompositions of the mixed crystals fall within the range of 0.98 to1.01. Since the composition of the perovskite oxide according to thepresent invention is determined to satisfy the inequalities (1), thecomposition of the perovskite oxide according to the present inventionis at or near the MPB.

TABLE 1 TF > 1 TF < 1 MPB Radi- Radi- Radi- Radi- Radi- Radi- Crystal usof us of Crystal us of us of us of us of System A ion B ion TF FractionSystem A ion B ion TF Fraction A ion B ion TF PT-PZ PbTiO₃ T 1.49 0.611.017 0.48 PbZrO₃ R 1.49 0.72 0.964 0.52 1.49 0.67 0.989 PT-PS PbTiO₃ T1.49 0.61 1.017 0.45 PbSnO₃ R 1.49 0.69 0.978 0.55 1.49 0.65 0.995PT-BiF PbTiO₃ T 1.49 0.61 1.017 0.3 BiFeO₃ R 1.37 0.55 1.005 0.7 1.410.57 1.008 BT-BiNT BT T 1.61 0.61 1.059 0.15 (Bi, Na)TiO₃ R 1.38 0.610.978 0.85 1.41 0.61 0.990 BT-BiNT BT T 1.61 0.61 1.059 0.07 (Bi,Na)TiO₃ R 1.38 0.61 0.978 0.93 1.40 0.61 0.984 BT-BiNT (Bi, K)TiO₃ T1.51 0.61 1.024 0.2 (Bi, Na)TiO₃ R 1.38 0.61 0.978 0.8 1.41 0.61 0.987KNN KNbO₃ O 1.64 0.64 1.054 0.49 NaNbO₃ O 1.39 0.64 0.967 0.51 1.51 0.641.010

For example, it is possible to design a perovskite oxide satisfying theconditions (1) to (4) (preferably, the conditions (1) to (5)) by firstselecting a first material having the composition ADO₃ and a tolerancefactor TF greater than 1.0 as the first component and a second materialhaving the composition BEO₃ and a tolerance factor TF smaller than 1.0as the second component, and then appropriately determining the additiveelements C and F so that the tolerance factor TF of the perovskite oxide(A, B, C) (D, E, F)O₃ is in the range of 0.98 to 1.01.

In the above design, the first component ADO₃ having a tolerance factorTF greater than 1.0 (i.e., exhibiting high ferroelectricity) and thesecond component BEO₃ having a tolerance factor TF smaller than 1.0(i.e., exhibiting high ferroelectricity) are selected, and the additiveelements C and F are determined so that the composition of theperovskite oxide (A, B, C) (D, E, F)O₃ is at or near the MPB. When thetolerance factor TF of the third component CFO₃ is near 1.0 (preferably,in the range of 0.98 to 1.02) and the ferroelectricity of the thirdcomponent CFO₃ is low, it is possible to design a perovskite oxidesatisfying the conditions (1) to (4) (preferably, the conditions (1) to(5)).

From the viewpoint of the piezoelectric (ferroelectric) performance ofthe perovskite oxide (A, B, C) (D, E, F)O₃ designed according to thepresent invention, it is more preferable that the ferroelectricity ofthe first component ADO₃ and the second component BEO₃ be higher. Thatis, it is more preferable that the tolerance factor TF of each of thefirst component ADO₃ and the second component BEO₃ more greatly differfrom 1.0.

Specifically, it is possible to bring the tolerance factor TF of theperovskite oxide (A, B, C) (D, E, F)O₃ into the range of 0.98 to 1.01 byselecting PT (PbTiO₃, which has the tolerance factor TF greater than1.0) and PZ (PbZrO₃, which has the tolerance factor TF smaller than 1.0)on the basis of the diagram of FIG. 1, and then adding, for example,niobium or the like as B-site ions. According to the diagram of FIG. 1,the ionic radii of the A-site ions in PbNbO₃, PT, and PZ are identical,and the tolerance factor TF of PbNbO₃ is between PT and PZ. Therefore,it is possible to bring the tolerance factor TF of the perovskite oxide(A, B, C) (D, E, F)O₃ into the range of 0.98 to 1.01 by adding niobiumto the mixed crystal of PT and PZ. In this case, the composition of theperovskite oxide becomes Pb(Ti, Zr, Nb)O₃.

In consideration of the valences of the ions, the crystal formed of onlyPbNbO₃ cannot have a perovskite structure. In the material designaccording to the present invention, the overall composition of theperovskite oxide is designed by theoretically obtaining the tolerancefactor TF of each of the oxides ADO3, BEO₃, and CFO₃ irrespectively ofwhether or not the crystal formed of only one of the oxides ADO3, BEO₃,and CFO₃ can have a perovskite structure.

In the case where PT (PbTiO₃, which has the tolerance factor TF greaterthan 1.0) and PZ (PbZrO₃, which has the tolerance factor TF smaller than1.0) are selected, alternatively, it is also possible to satisfy theconditions (1) to (4) (preferably, the conditions (1) to (5)) by addingas B-site ions other elements each having an ionic radius near to theionic radius of niobium, such as Sn, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn,Co, Ni, V, and Fe.

Further, it is possible to add two or more of Sn, Nb, Ta, Mo, W, Ir, Os,Pd, Pt, Re, Mn, Co, Ni, V, and Fe as B-site ions. That is, thecomposition of the above example of the perovskite oxide according tothe present invention can be expressed by the compositional formula,Pb(Ti, Zr, M)O₃,  (PX)where M in the compound Pb(Ti, Zr, M)O₃ is at least one of the metalelements Sn, Nb, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn, Co, Ni, V, and Fe.

Alternatively, it is possible to add an element as A-site ions (as theelement C in the composition (A, B, C) (D, E, F)O₃) to a mixed crystalof PT and PZ, instead of adding an element as the B-site ions (as theelement F in the composition (A, B, C)(D, E, F) O₃). Furtheralternatively, it is possible to add both of an element as A-site ions(as the element C in the composition (A, B, C)(D, E, F) O₃) and anelement as B-site ions (as the element F in the composition (A, B, C)(D, E, F)O₃) to a mixed crystal of PT and PZ.

In addition, it is also possible to bring the tolerance factor TF of theperovskite oxide (A, B, C) (D, E, F)O₃ into the range of 0.98 to 1.01 byselecting BT (BaTiO₃, which has the tolerance factor TF greater than1.0) and CT (CaTiO₃, which has the tolerance factor TF smaller than 1.0)on the basis of the diagram of FIG. 1, and then adding, for example,strontium or the like as A-site ions. In this case, the composition ofthe perovskite oxide becomes (Ba, Ca, Sr)TiO₃.

Further, it is also possible to apply the above manner of designing theA-site ions (Ba, Ca, and Sr) to the aforementioned mixed crystalcontaining Ti and Zr as B-site ions, and further add the element M asB-site ions to the mixed crystal containing Ti and Zr as B-site ions andBa, Ca, and Sr as the A-site elements (in a similar manner to theaforementioned manner of designing the B-site elements in Pb (Ti, Zr,M)O₃). In this case, the composition of the perovskite oxide can beexpressed by the compositional formula,(Ba, Ca, Sr)(Ti, Zr, M)O₃,  (PY)where M in the compound (Ba, Ca, Sr)(Ti, Zr, M)O₃ is at least one of themetal elements Sn, Nb, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn, Co, Ni, V, andFe.

Furthermore, it is also possible to bring the tolerance factor TF of theperovskite oxide (A, B, C) (D, E, F)O₃ into the range of 0.98 to 1.01 byselecting BA (BiAlO₃, which has the tolerance factor TF greater than1.0) as the first component ADO₃ and BF (BiFeO₃, which has the tolerancefactor TF smaller than 1.0) as the second component BEO₃ on the basis ofthe diagram of FIG. 1, and then adding, for example, strontium or thelike as B-site ions. In this case, since TF(BiFeO₃)=0.960 andTF(BiScO₃)=0.911 as indicated in FIG. 1, TF(BiScO₃)<TF(BiFeO₃), andBiScO₃ does not satisfy the inequalities (5). However, it is possible tobring the tolerance factor TF of the perovskite oxide Bi(Al,Fe,Sc)O₃into the range of 0.98 to 1.01 by appropriately adjusting thecomposition. (For example, the tolerance factor TF ofBi(Al_(0.6)Fe_(0.35)Sc_(0.05))O₃ is 0.989.) The perovskite oxideBi(Al,Fe,M)O₃ can also satisfy the inequalities (1) to (4) (and theinequalities (5) in preferable cases) in the case where the element Madded as B-site ions in the perovskite oxide Bi(Al,Fe,M)O₃ is an element(such as Cr, Mn, Co, Ni, or Ga) having an ionic radius near to the ionicradius of scandium. Thus, the composition of the perovskite oxide can beexpressed by the compositional formula,Bi(Al,Fe,M)O₃,  (PW)where M in the compound Bi (Al,Fe,M)O₃ is at least one of the metalelements Cr, Mn, Co, Ni, Ga, and Sc.

According to the present invention, the tolerance factor of each of theoxides (A, B, C) (D, E, F)O₃, ADO₃, BEO₃, and CFO₃ is theoreticallyobtained, and then the composition of the perovskite oxide (A, B, C) (D,E, F)O₃ is determined. At this time, even when one or more of the oxidesdo not form a perovskite structure by themselves, the tolerance factorof each of the oxides is theoretically obtained, and then thecomposition of the perovskite oxide (A, B, C) (D, E, F)O₃ is obtained soas to satisfy the conditions (1) to (4) (preferably, the conditions (1)to (5)). The perovskite oxides designed in accordance with the materialdesign principle as above have composition at or near the MPB, andtherefore exhibit high piezoelectric (ferroelectric) performance.

As explained in the “SUMMARY OF THE INVENTION,” the phase structure ofthe perovskite oxide is not specifically limited. For example, theperovskite oxide may have a three-phase mixed-crystal structure in whichthe three components ADO₃, BEO₃, and CFO₃ coexist, or a single-phasestructure in which the three components ADO₃, BEO₃, and CFO₃ arecompletely solid solved into a single phase, or another structure.

It is preferable that the perovskite oxide according to the presentinvention contain the first component ADO₃ having the tolerance factorTF greater than 1.0, the second component BEO₃ having the tolerancefactor TF smaller than 1.0, and the third component CFO₃ having thetolerance factor TF near to 1.0. As mentioned before, in each of thefirst component ADO₃, the second component BEO₃, and the third componentCFO₃, the ratio of each of the molar amount of the A-site element andthe molar amount of the B-site element to the molar amount of oxygenatoms is normally 1:3. However, the ratio of each of the molar amount ofthe A-site element and the molar amount of the B-site element to themolar amount of oxygen atoms may deviate from 1:3 within a range inwhich each of the first component ADO₃, the second component BEO₃, andthe third component CFO₃ can form a perovskite structure.

It is also preferable that the first component ADO₃ and the secondcomponent BEO₃ form structures respectively corresponding to differentcrystal systems, and particularly preferable that the first componentADO₃, the second component BEO₃, and the third component CFO₃ formstructures respectively corresponding to different crystal systems.

In a preferable example of the perovskite oxide, the crystal system ofthe first component ADO₃ is one of the tetragonal, orthorhombic,monoclinic, trigonal, and rhombohedral systems, the crystal system ofthe second component BEO₃ is one of the tetragonal, orthorhombic, andrhombohedral systems which is different from the first component ADO₃,and the third component CFO₃ is one of the cubic and pseudocubicsystems.

When a perovskite oxide is designed according to the present invention,it is possible to produce the perovskite oxide having a composition ator near the MPB and a mixed-crystal structure composed of at least twocrystal phases selected from the tetragonal, orthorhombic, andrhombohedral phases and at least one crystal phase selected from thecubic and pseudocubic phases.

In addition, when a perovskite oxide is designed according to thepresent invention, it is also possible to produce the perovskite oxidehaving a composition at or near the MPB, and exhibiting a firstdiffraction peak of a tetragonal phase, a second diffraction peak of arhombohedral phase, and a third diffraction peak different from thefirst and second diffraction peaks in a high-resolution X-raydiffraction (XRD) profile.

The present inventor has performed measurement of high-resolution XRD,and confirmed that the Nb-doped PZT (Nb-PZT) having composition at ornear the MPB can form a three-phase mixed-crystal structure in which thetetragonal and rhombohedral phases and a third phase different from thetetragonal and rhombohedral phases coexist (as explained later as theconcrete example 1 with reference to FIG. 9A). Nb-PZT is one of theoxides which can be expressed by the compositional formula Pb (Ti, Zr,M)O₃, and is a perovskite oxide which satisfies the aforementionedconditions (1) to (5). On the other hand, no diffraction peakcorresponding to the third phase has been found in undoped PZT havingcomposition at or near the MPB (as explained later as the comparisonexample 1 with reference to FIG. 9B).

The present inventor has confirmed the fact that the tolerance factor TFof the third component is near to 1.0, and the lattice constant of thephase of the third component obtained from the corresponding diffractionpeak in the high-resolution XRD coincides with the lattice constantwhich is expected when the phase of the third component is assumed to bethe cubic or pseudocubic phase. The present inventor conjectures thatthe third phase is a cubic or pseudocubic phase, on the basis of theabove fact and a result of EXAFS (Extended X-ray Absorption FineStructure) measurement (as explained later as the concrete example 1with reference to FIG. 9A and Table 2).

The present inventor has confirmed that the perovskite oxide accordingto the present invention having a three-phase mixed-crystal structure isa material which exhibits high piezoelectric performance, and can beeffectively used in the system utilizing phase transition induced by anelectric field as proposed in WO2007/034903.

2. Characteristics of Piezoelectric Body

Hereinbelow, the system utilizing phase transition induced by anelectric field as proposed in WO2007/034903 is explained. This systemincludes a piezoelectric body having a first ferroelectric phase whichforms a structure in a first crystal system. When an electric field isapplied to the piezoelectric body, at least a portion of the firstferroelectric phase transitions to a second ferroelectric phase whichforms a structure in a second crystal system different from the firstcrystal system.

First, the piezoelectric characteristic of the above piezoelectric bodyis explained below, where it is assumed, for simple explanation, thatthe entire piezoelectric body is initially in the first ferroelectricphase, which transitions to the second ferroelectric phase when anelectric field is applied to the piezoelectric body, and the structuresin the first and second ferroelectric phases correspond to differentcrystal systems. FIG. 2 schematically indicates, by the thick solidpolygonal line X, a relationship between the strength of the electricfield and the distortion amount in the piezoelectric body.

In FIG. 2, E1 is the minimum electric field strength at which the phasetransition from the above first ferroelectric phase begins, and E2 isthe electric field strength at which the phase transition issubstantially completed. The “electric field strength at which the phasetransition is substantially completed” is such a level of the electricfield strength that the phase transition no longer occurs even when theelectric field strength becomes higher than the level. In some cases,even when the electric field strength becomes higher than E2, the phasetransition does not occur in a portion of the first ferroelectric phase,so that the portion in the first ferroelectric phase remains.

In the first range of the electric field strength E of 0 to E1, thedistortion amount of the piezoelectric body linearly increases withincrease in the electric field strength because of the piezoelectriceffect in the first ferroelectric phase (before the phase transition).In the second range of the electric field strength E of E1 to E2, thedistortion amount of the piezoelectric body still linearly increaseswith increase in the electric field strength because the change in thecrystal structure associated with the phase transition causes a volumechange in the piezoelectric body. In the third range of the electricfield strength E greater than E2, the distortion amount of thepiezoelectric body further linearly increases with increase in theelectric field strength because of the piezoelectric effect in thesecond ferroelectric phase (after the phase transition).

As explained above, a change in the crystal structure associated withphase transition causes a volume change in the piezoelectric body, andthe piezoelectric effect in the ferroelectric material works both beforeand after the phase transition since the piezoelectric body is in aferroelectric phase both before and after the phase transition.Therefore, the piezoelectric body according to the present invention canachieve great distortion in each of the first range of 0 to E1, thesecond range of E1 to E2, and the third range greater than E2.

Further, FIG. 2 also schematically indicates the piezoelectriccharacteristic of the piezoelectric body in the first type ofconventional piezoelectric devices as disclosed in JP2006-036578, andthe piezoelectric characteristic of the piezoelectric body in the secondtype of conventional piezoelectric devices as disclosed in JP3568107.Specifically, the piezoelectric characteristic of the piezoelectric bodyin the first type of conventional piezoelectric devices is basically thesame as the piezoelectric characteristic of the piezoelectric body inthe piezoelectric device as proposed in WO2007/034903 in the first rangeof the electric field strength (0≦E≦E1), and different from thepiezoelectric characteristic of the piezoelectric body in thepiezoelectric device as proposed in WO2007/034903 in the second andthird ranges of the electric field strength (E1≦E), and the portion ofthe piezoelectric characteristic of the piezoelectric body in the firsttype of conventional piezoelectric devices in the second and thirdranges of the electric field strength (E1≦E) is indicated by the thickone-dot chain line Y. In addition, the piezoelectric characteristic ofthe piezoelectric body in the second type of conventional piezoelectricdevices is basically the same as the piezoelectric characteristic of thepiezoelectric body in the piezoelectric device as proposed inWO2007/034903 in the first and second ranges of the electric fieldstrength (0≦E≦E2), and different from the piezoelectric characteristicof the piezoelectric body in the piezoelectric device as proposed inWO2007/034903 in the second and third ranges of the electric fieldstrength (E2≦E), and the portion of the piezoelectric characteristic ofthe piezoelectric body in the second type of conventional piezoelectricdevices in the third range of the electric field strength (E2≦E) isindicated by the thick two-dot chain solid line Z.

As mentioned before in the “Description of the Related Art”, in theaforementioned first type of conventional piezoelectric devices, thepiezoelectric effect of expansion in the direction of the spontaneouspolarization is generally utilized by applying an electric field to aferroelectric body along the direction of the spontaneous polarization.That is, in the first type of conventional piezoelectric devices, thedistortion amount of the piezoelectric body linearly increases withincrease in the electric field strength until the electric fieldstrength reaches a predetermined level. However, when the electric fieldstrength exceeds the predetermined level, the amount of increase in thedistortion is substantially reduced and the distortion amount is almostsaturated.

In addition, as mentioned before in the “Description of the RelatedArt”, JP3568107 discloses substantially only the piezoelectric bodyutilizing the phase transition between a ferroelectric phase and aparaelectric phase. In the piezoelectric devices disclosed in JP3568107,the distortion amount of the piezoelectric body linearly increases withincrease in the electric field strength because of the piezoelectriceffect in the first ferroelectric phase (before the phase transition)until the electric field strength reaches a level at which the phasetransition begins (as the electric field strength E1 in FIG. 2). Then,the distortion amount of the piezoelectric body further linearlyincreases with increase in the electric field strength because thechange in the crystal structure associated with the phase transitioncauses the volume change in the piezoelectric body until the electricfield strength reaches a level at which the phase transition issubstantially completed (as the electric field strength E2 in FIG. 2).However, when the electric field strength exceeds the level at which thephase transition is substantially completed (as the electric fieldstrength E2 in FIG. 2), the piezoelectric effect does not work, so thatthe distortion amount does not further increase.

On the other hand, the piezoelectric body in which a first ferroelectricphase corresponding to a first crystal system transitions to a secondferroelectric phase corresponding to a second crystal system when anelectric field is applied to the piezoelectric body can achieve greaterdistortion amount than the piezoelectric bodies in the aforementionedfirst and second types of conventional piezoelectric devices. Althoughno specific condition is imposed on the activation of the piezoelectricbody, in consideration of the distortion amount, it is preferable toactivate the piezoelectric body so that the minimum electric fieldstrength Emin and the maximum electric field strength Emax satisfy thecondition expressed by the inequalities (6).Emin<E1<Emax  (6)In addition, it is particularly preferable to activate the piezoelectricbody so that the minimum electric field strength Emin and the maximumelectric field strength Emax satisfy the condition expressed by theinequalities (7).Emin<E1≦E2<Emax  (7)

Further, it is preferable that the ferroelectric phase in which thephase transition occurs have crystal orientation along a directiondifferent from the orientation of the spontaneous polarization, and itis more preferable that the ferroelectric phase after the phasetransition has crystal orientation along a direction approximatelyidentical to the orientation of the spontaneous polarization axis afterthe phase transition. Normally, the crystal orientation is equal to thedirection of the applied electric field. It is particularly preferableto approximately equalize the direction of the applied electric fieldwith the orientation of the spontaneous polarization axis after thephase transition, since in this case the engineered-domain effect canwork before the phase transition and make the distortion amount beforethe phase transition greater than the distortion amount achieved byequalizing the direction of the applied electric field with theorientation of the spontaneous polarization axis before the phasetransition. The engineered-domain effect in a monocrystal is explainedby S. E. Park et al., “Ultrahigh strain and piezoelectric behavior inrelaxor based ferroelectric single crystals”, Journal of AppliedPhysics, Vol. 82, pp. 1804-1811, 1997.

Furthermore, the phase transition can readily occur when the directionof the applied electric field is approximately equal to the orientationof the spontaneous polarization axis after the phase transition. Thepresent inventor considers that since the state in which the directionof the applied electric field is equal to the orientation of thespontaneous polarization axis is crystallographically stable, transitionto the more stable state can readily occur. In some cases, phasetransition does not occur in a portion of the ferroelectric phase evenwhen the electric field higher than the electric field strength E2 isapplied to the piezoelectric body. However, when the phase transitioncan readily occur, it is possible to reduce the portion theferroelectric phase in which the phase transition does not occur evenwhen the electric field higher than the electric field strength E2 isapplied to the piezoelectric body. Therefore, in the case where thedirection of the applied electric field is approximately equal to theorientation of the spontaneous polarization axis after the phasetransition, it is possible to stably achieve greater distortion than inthe case where the direction of the applied electric field is equal tothe orientation of the spontaneous polarization axis before the phasetransition.

Moreover, since the direction of the applied electric field isapproximately equal to the orientation of the spontaneous polarizationaxis after the phase transition, the piezoelectric effect effectivelyworks in the ferroelectric phase after the phase transition, so that itis possible to stably achieve great distortion.

As explained above, in the case where the direction of the appliedelectric field is approximately equal to the orientation of thespontaneous polarization axis after the phase transition, it is possibleto achieve great distortion before, during, and after the phasetransition. This effect works at least when the direction of the appliedelectric field is different from the orientation of the spontaneouspolarization axis before the phase transition, and becomes moreprominent when the direction of the applied electric field approachesthe orientation of the spontaneous polarization axis after the phasetransition.

In the above explanations, the piezoelectric body is assumed to have asingle-phase structure containing only a single first ferroelectricphase corresponding to a first crystal system which transitions to asecond ferroelectric phase corresponding to a second crystal systemdifferent from the first crystal system when an electric field isapplied to the piezoelectric body. However, the above explanations canalso be applied to piezoelectric bodies which have a mixed-crystalstructure containing a ferroelectric phase corresponding to a crystalsystem which transitions to another ferroelectric phase corresponding toa different crystal system when an electric field is applied to thepiezoelectric body.

3. Phase-Transition Model 1

The present inventor has found that the perovskite oxide according tothe present invention having a three-phase mixed-crystal structurecomposed of the first component ADO₃, the second component BEO₃, and thethird component CFO₃ can be effectively used in the aforementionedsystem utilizing phase transition induced by application of an electricfield. Hereinbelow, a model of phase transition in the three-phasemixed-crystal structure is explained as a phase transition model 1 withreference to FIGS. 3A, 3B, and 3C, which illustrate exemplary states ofthe three-phase mixed-crystal structure constituted by domainscorresponding to three crystal systems in the phase transition model 1.In the phase transition model 1, the first component forms a tetragonalcrystal (T), the second component forms a rhombohedral crystal (R), andthe third component forms a pseudocubic crystal (PC).

Although the ferroelectric phase of one or each of the first and secondcomponents can transition to a different ferroelectric phasecorresponding to a different crystal system, a case where the phase ofthe second component transitions from the rhombohedral phase (R) to atetragonal phase (T) is taken as an example in the followingexplanation. In addition, in the following example, the ferroelectricphase of the first component has crystal orientation along theorientation of the spontaneous polarization axis (i.e., the electricfield is applied along the orientation of the spontaneous polarizationaxis of the ferroelectric phase of the first component), and theferroelectric phase of the second component has crystal orientationapproximately along the orientation of the spontaneous polarization axisafter the phase transition (i.e., the orientation of the appliedelectric field is approximately identical to the orientation of thespontaneous polarization axis of the ferroelectric phase of the secondcomponent after the phase transition). In FIGS. 3A, 3B, and 3C, thearrowed solid lines indicate the directions of the polarizations in therespective domains, and the hollow arrows bearing the reference “E”indicate the direction and the electric field strengths of the appliedelectric field.

As indicated in FIG. 3A, when no electric field is applied to apiezoelectric body of the perovskite oxide having the above three-phasemixed-crystal structure (i.e., when E=0), the tetragonal domains D1 ofthe first component and the rhombohedral domains D2 of the secondcomponent exhibit ferroelectricity, while the pseudocubic domains D3 ofthe third component do not exhibit ferroelectricity.

The present inventor considers that when the electric field E at a lowelectric field strength not exceeding the aforementioned minimumelectric field strength E1 (at which the phase transition of theferroelectric domains begins) is applied to the piezoelectric asillustrated in FIG. 3B, first, the pseudocubic domains D3 of the thirdcomponent transition to ferroelectric domains having spontaneouspolarization axes along the direction of the applied electric field. Inthe example illustrated in FIG. 3B, the entire pseudocubic domains D3 ofthe third component transition to tetragonal domains (T). However, insome cases, a portion of the pseudocubic domains D3 may not transition,and may remain pseudocubic.

The present inventor considers that the displacement in the pseudocubicdomains D3 of the third component caused by the transition of thedomains D3 induces and facilitates transition of the tetragonal domainsD1 of the first component and the domains D2 of the second component(which abut the domains D3).

In the rhombohedral domains D2 of the second component in which TF<1.0,the B-site ions are greater than the A-site ions, so that the B-siteions cannot easily move. On the other hand, in the tetragonal domains D1of the first component in which TF>1.0, the B-site ions are smaller thanthe A-site ions, so that the B-site ions can relatively easily move.Therefore, the present inventor considers that the displacement in thedomains D3 of the third component caused by the transition of thedomains D3 induces the transition of the tetragonal domains D1 of thefirst component more than the transition of the rhombohedral domains D2of the second component at the low electric field strength, andtherefore the domains D1 can expand along the orientation of thespontaneous polarization axis more easily than the domains D2.

The present inventor also considers that when the electric fieldstrength is further increased, the displacement in the pseudocubicdomains D3 of the third component caused by the transition of thedomains D3 and/or the displacement in the tetragonal domains D1 of thefirst component caused by the transition of the domains D1 induce andfacilitate transition of the rhombohedral domains D2 of the secondcomponent to the tetragonal phase (T). FIG. 3C shows a state of thepiezoelectric body to which an electric field with the strength E higherthan the electric field strength E2 (at which the phase transition ofthe ferroelectric phases is substantially completed) is applied.Although the entire domains D2 have transitioned to the tetragonal phase(T) in the example illustrated in FIG. 3C, in some cases, a portion ofthe domains D2 may not transition, and may remain rhombohedral.

FIG. 4A is a diagram schematically indicating the relationship betweenthe potential energy and the distance from the center to the B-siteelement in the unit cell of the rhombohedral crystal, and FIG. 4B is adiagram schematically indicating the relationship between the potentialenergy and the distance from the center to the B-site element in theunit cell of the cubic or pseudocubic crystal. As indicated in FIG. 4A,the potential energy of the rhombohedral (R) crystal is lowest (i.e.,the rhombohedral crystal is most stable) when the B-site ions are at twopositions slightly apart from the centers of the unit cells. On theother hand, as indicated in FIG. 4B, the potential energy of the cubic(C) or pseudocubic (PC) crystal is lowest (i.e., the cubic orpseudocubic crystal is most stable) when the B-site ions are at thecenters of the unit cells.

In addition, FIG. 4B shows that the slope of the potential energy curvefor the cubic (C) or pseudocubic (PC) crystal is relatively gentle whenthe B-site ions move slightly away from the centers of the unit cells.Therefore, it is possible to consider that the B-site ions in the cubic(C) or pseudocubic (PC) crystal can move relatively easily at relativelylow electric field strength, and therefore phase transition can occurrelatively easily in the cubic (C) or pseudocubic (PC) crystal. On theother hand, FIG. 4A shows that the slope of the potential energy curvefor the rhombohedral (R) crystal is relatively steep when the B-siteions move slightly away from the centers of the unit cells. Therefore,it is possible to consider that the B-site ions in the rhombohedral (R)crystal can move less easily than in the cubic (C) or pseudocubic (PC)crystal, and therefore phase transition can occur less easily in therhombohedral (R) crystal than in the cubic (C) or pseudocubic (PC)crystal. Nevertheless, the present inventor considers that the existenceof the domains D3 of the third component, the phase of which transitionsfirst, facilitates the phase transition in the rhombohedral (R) crystal.

The system in which phase transition is induced by application of anelectric field according to the present invention can achieve higherpiezoelectric performance than the conventional systems in which nophase transition occurs or only the phase transition from aferroelectric phase to a paraelectric phase occurs. In addition, thepresent inventor considers that the existence of the domains D3 of thethird component, the phase of which transitions first, facilitates thephase transition in the ferroelectric domains D1 of the first componentand the ferroelectric domains D2 of the second component, and thereforehigher piezoelectric performance can be achieved even at relatively lowelectric field strength.

Although the phase of the domains D3 of the third component isconsidered to transition first as explained above, the domains D3 of thethird component are considered to be paraelectric when no electric fieldis applied to the piezoelectric body. Therefore, the minimum electricfield strength E1 (at which the phase transition of the ferroelectricphase begins) is the minimum electric field strength at which the phasetransition begins in either of the ferroelectric domains D1 of the firstcomponent and the ferroelectric domains D2 of the second component.

As explained later on the basis of the concrete example 2 and thecomparison example 2, the present inventor has confirmed that Nb-dopedPZT (Nb-PZT) having a three-phase mixed-crystal structure with acomposition of PT (as the first component), PZ (as the secondcomponent), and PbNbO₃ (as the third component) at or near the MPB canrealize a high piezoelectric coefficient even at relatively low electricfield strength, compared with undoped PZT having a two-phasemixed-crystal structure with a composition of only PT (as the firstcomponent) and PZ (as the second component) at or near the MPB.

4. Phase-Transition Model 2

Even in the case where the perovskite oxides designed according to thepresent invention have a phase structure other than the three-phasemixed-crystal structure, the perovskite oxides can be effectively usedin the system utilizing phase transition induced by application of anelectric field. The present inventor has produced a film of Nb-doped PZT(Nb-PZT) having a composition at or near the MPB and a phase structurebeing formed mainly of a rhombohedral phase and containing sometetragonal phase, and has confirmed that the film of Nb-PZT can beeffectively used in the system utilizing phase transition induced byapplication of an electric field, and can have a high piezoelectriccoefficient even at relatively low electric field strength, as explainedlater as the concrete example 3. Hereinbelow, a model of phasetransition in the above phase structure is explained as a phasetransition model 2 with reference to FIGS. 5A, 5B, and 5C, whichillustrate exemplary states of the mixed-crystal structure constitutedby domains corresponding to two crystal systems in the phase transitionmodel 2. In the phase transition model 2, the piezoelectric body iscomposed mainly of a rhombohedral phase (R) when no electric field isapplied, and the rhombohedral phase (R) transitions to a tetragonalphase (T) when an electric field is applied. In addition, in thefollowing example, the ferroelectric phase (R) has crystal orientationapproximately along the orientation of the spontaneous polarization axisof the tetragonal phase (T) (i.e., the orientation of the appliedelectric field is approximately identical to the orientation of thespontaneous polarization axis of the ferroelectric phase after the phasetransition).

The present inventor considers that the phase transition in the phasetransition model 2 progresses as follows.

When no electric field is applied to a piezoelectric body of theperovskite oxide having the above phase structure (i.e., when E=0), thepiezoelectric body is formed mainly of a rhombohedral domain (R), andtetragonal nanodomains (T) are formed in the rhombohedral domain (R) asillustrated in FIG. 5A due to the doping with the elements C and F inthe compound (A, B, C)(D, E, F)O₃.

The above tetragonal nanodomains (T) become the seeds (starting points)of the phase transition. That is, when an electric field with anintermediate strength E equal to or higher than the minimum electricfield strength E1 (at which the phase transition of the ferroelectricdomain (R) begins) is applied to the piezoelectric body which initiallyhas the phase structure of FIG. 5A, i.e., when E1≦E≦E2, the phasetransition of the rhombohedral domain (R) develops from the tetragonalnanodomains (T), so that larger tetragonal domains (T) than thenanodomains are formed around the nanodomains as illustrated in FIG. 5B.

Thereafter, when the electric field strength is further increased, theabove phase transition to the larger tetragonal domains (T) induces andfacilitates displacement in and phase transition of the remainingportion of the ferroelectric domain (R), and resultantly produces greatdistortion induced by an electric field. FIG. 5C shows a state of thepiezoelectric body when an electric field with the strength not lowerthan the electric field strength E2 (at which the phase transition ofthe ferroelectric domain (R) is substantially completed) is applied tothe piezoelectric body, i.e., when E≧E2. Although the entirerhombohedral domain (R) has transitioned to the tetragonal phase (T), insome cases, a portion of the rhombohedral domain (R) may not transition,and remain rhombohedral.

As explained above, the present invention presents a novel materialdesign principle for producing a perovskite oxide superior inpiezoelectric (ferroelectric) performance. Therefore, according to thepresent invention, it is possible to easily design the composition of aperovskite oxide at or near the MPB so that the perovskite oxideexhibits superior piezoelectric (ferroelectric) performance.

In particular, the perovskite oxide produced in accordance with thematerial design principle of the present invention is suitable for usein the system utilizing phase transition induced by application of anelectric field as proposed in WO2007/034903. According to the presentinvention, it is possible to provide a perovskite oxide having a domainstructure in which phase transition can readily occur and greatdistortion can be achieved even at relatively low electric fieldstrength. The feature of achieving great distortion at relatively lowelectric field strength is preferable for saving energy.

5. Ferroelectric Compound

As described before, the ferroelectric compound according to the presentinvention is characterized in containing the perovskite oxide thecomposition of which is designed in accordance with the material designprinciple according to the present invention. The ferroelectric compoundaccording to the present invention can further contain an arbitrarycomponent other than the perovskite oxide according to the presentinvention, such as a perovskite oxide other than the perovskite oxideaccording to the present invention, another dopant, a sintering agent,and the like.

6. Piezoelectric Device and Inkjet Recording Head

As described before, the piezoelectric device according to the presentinvention is characterized in comprising the piezoelectric bodyaccording to the present invention and electrodes for applying anelectric field to the piezoelectric body. Since the piezoelectric deviceaccording to the present invention uses the perovskite oxide accordingto the present invention, the piezoelectric device according to thepresent invention exhibits high piezoelectric performance.

For example, in the case where the perovskite oxide according to thepresent invention used in a piezoelectric body in the piezoelectricdevice has a three-phase mixed-crystal structure composed of the firstcomponent ADO₃ with the tolerance factor TF greater than 1.0, the secondcomponent BEO₃ with the tolerance factor TF smaller than 1.0, and thethird component CFO₃ with the tolerance factor TF near to 1.0, and theferroelectric phase of one or each of the first and second componentstransitions to another ferroelectric phase having a different crystalstructure in response to application of an electric field to thepiezoelectric body, the piezoelectric device exhibits high piezoelectricperformance even at relatively low electric field strength. Hereinbelow,the structures of the above piezoelectric device and an inkjet recordinghead (as a liquid discharge device) using the piezoelectric device areexplained with reference to FIG. 6, which schematically shows a crosssection of an essential portion of the inkjet recording head containingthe above piezoelectric device, where the cross section is parallel tothe thickness direction of the piezoelectric device. In FIG. 6, therespective elements are illustrated schematically, and the dimensions ofthe illustrated elements are different from the dimensions of theelements of actual systems.

As illustrated in FIG. 6, the inkjet recording head 3 contains apiezoelectric actuator 2, which is realized by using the piezoelectricdevice 1. The piezoelectric device 1 is a device produced by forming ona substrate 11 a lower electrode 12, a piezoelectric body 13, and anupper electrode 14 in this order. The piezoelectric body 13 is apolycrystalline body formed of a perovskite oxide which is designed inaccordance with the material design principle according to the presentinvention as explained before, although the piezoelectric body maycontain inevitable impurities.

The material for the substrate 11 is not specifically limited. Forexample, the substrate 11 may be made of silicon, glass, stainlesssteel, YSZ (yttrium stabilized zirconia), alumina, sapphire, siliconcarbide, or the like. In addition, the substrate 11 may be realized by alaminated substrate such as the SOI (silicon-on-insulator) substrate,which is produced by forming on a surface of a silicon substrate anoxide film of SiO₂ and a Si active layer in this order.

In addition, the main component of the lower electrode 12 is notspecifically limited, and may be, for example, one or a combination ofmetals such as Au, Pt, and Ir and metal oxides such as IrO₂, RuO₂,LaNiO₃, and SrRuO₃. Further, the main component of the upper electrodes14 is not specifically limited, and may be, for example, one or acombination of the same materials as the examples of the main componentof the lower electrode 12 and other materials (such as Al, Ta, Cr, orCu) which are generally used in the electrodes in the semiconductorprocesses. Moreover, the thicknesses of the lower and upper electrodes12 and 14 are not specifically limited, and are preferably 50 to 500 nm.

The piezoelectric actuator 2 comprises a diaphragm 16 and a controller15 as well as the piezoelectric device 1. The diaphragm 16 is attachedto the back surface of the substrate 11 so that the diaphragm 16vibrates in correspondence with expansion and contraction of thepiezoelectric body 13. The controller 15 includes a driver circuit andthe like for driving the piezoelectric device 1.

The inkjet recording head 3 is produced by attaching an ink nozzle 20 tothe back surface of the piezoelectric actuator 2. The ink nozzle 20 is amember for reserving and discharging ink, and comprises an ink chamber21 (as a liquid-reserve chamber) and an ink outlet 22 (as aliquid-discharge outlet). The ink chamber 21 reserves ink, and the inkheld in the ink chamber 21 is discharged out of the ink chamber 21through the ink outlet 22 connected to the ink chamber 21.

In the above inkjet recording head 3, the strength of the electric fieldapplied to the piezoelectric device 1 is increased or decreased so as toexpand or contract the piezoelectric element and control the dischargeand the discharge amount of the ink.

Alternatively, it is possible to process portions of the substrate 10into the diaphragm 16 and the ink nozzle 20, instead of attaching thediaphragm 16 and the ink nozzle 20 to the piezoelectric device 1. Forexample, in the case where the substrate 11 is realized by a laminatedsubstrate such as an SOI substrate, the ink chambers 21 can be formed byetching the corresponding portions of the bottom surface of thesubstrate 11, and the diaphragm 16 and the other structures of the inknozzle 20 can be formed by processing the substrate 11 per se.

The form of the piezoelectric body 13 can be designed as appropriate.For example, the piezoelectric body 13 may have the form of a film or asintered ceramic body. In the field of the inkjet recording heads andthe like, techniques for more densely arranging piezoelectric elements(devices) are currently being studied in order to improve image quality.In association with the increase in the density of the arrangement ofpiezoelectric elements, techniques for reduction in the thicknesses ofthe piezoelectric devices are also being studied. In order to reduce thethickness of the piezoelectric device, the piezoelectric body 13 ispreferably a piezoelectric film, and more preferably a thinpiezoelectric film having the thickness of 20 nm or smaller. Since thethin piezoelectric film is required to have a high piezoelectriccoefficient, and the perovskite oxide according to the present inventionhas a high piezoelectric coefficient, the perovskite oxide according tothe present invention can be effectively used as the material for thethin piezoelectric film.

According to the present embodiment, the piezoelectric body 13 may havea three-phase mixed-crystal structure composed of the first componentADO₃ with the tolerance factor TF greater than 1.0, the second componentBEO₃ with the tolerance factor TF smaller than 1.0, and the thirdcomponent CFO₃ with the tolerance factor TF near to 1.0, and theferroelectric phase of one or each of the first and second componentstransitions to another ferroelectric phase having a different crystalstructure in response to application of an electric field to thepiezoelectric body.

For example, the piezoelectric body 13 has the composition expressed bythe compositional formula (PX) or (PY).Pb(Ti,Zr,M)O₃,  (PX)(Ba,Ca,Sr)(Ti,Zr,M)O₃,  (PY)In the compositional formula (PX) or (PY), M is at least one of themetal elements Sn, Nb, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn, Co, Ni, V, andFe.

According to the present embodiment, preferably, the ferroelectric phaseor phases which transition in response to application of the electricfield have crystal orientation. More preferably, the crystal orientationof each ferroelectric phase which transitions is along a directiondifferent from the orientation of the spontaneous polarization axis inthe ferroelectric phase, and particularly preferably, the crystalorientation is approximately identical to the orientation of thespontaneous polarization axis after the phase transition. According tothe present embodiment, the crystal orientation is identical to thedirection of the electric field applied to the piezoelectric body.

The spontaneous polarization axis of the ferroelectric material is <001>in the tetragonal system, <110> in the orthorhombic system, and <111> inthe rhombohedral system. It is possible to equalize the direction of theelectric field applied to the piezoelectric body with the spontaneouspolarization axis after the phase transition when the ferroelectricphase of the one or each of the first and second components is one ofthe rhombohedral phase having crystal orientation along approximatelythe <100> direction, the rhombohedral phase having crystal orientationalong approximately the <110> direction, the tetragonal phase havingcrystal orientation along approximately the <110> direction, thetetragonal phase having crystal orientation along approximately the<111> direction, the orthorhombic phase having crystal orientation alongapproximately the <100> direction, and the orthorhombic phase havingcrystal orientation along approximately the <111> direction.

For example, the piezoelectric body 13 having the crystal orientationmay be an orientated film (having single-axis orientation), an epitaxialfilm (having three-axis orientation), or a grain-oriented, sinteredceramic body. The orientated film can be formed under a conditionenabling production of a single-axis oriented crystal, by using one ofthe known thin-film formation techniques including the vapor-phasetechniques and the liquid-phase techniques, where the vapor-phasetechniques include sputtering, MOCVD (metal organic chemical vapordeposition), pulsed-laser deposition, and the like, and the liquid-phasetechniques include the sol-gel technique, MOD (metal organicdecomposition), and the like. For example, the (100) orientation can berealized by using (100)-oriented platinum or the like as the lowerelectrode. The epitaxial film can be formed by using, in the substrateand the lower electrode, materials which lattice-match well with thepiezoelectric film. For example, preferable combinations of thematerials for the substrate and the lower electrode are SrTiO₃/SrRuO₃,MgO/Pt, and the like. The grain-oriented, sintered ceramic body can beformed by the hot-pressing technique, the sheet technique, thelamination press technique, and the like.

The condition for driving the piezoelectric body 13 by the controller 15is not specifically limited. However, as explained before with referenceto FIG. 2, in consideration of the distortion amount, the piezoelectricbody 13 is preferably driven by the controller 15 so that the minimumelectric field strength Emin and the maximum electric field strengthEmax satisfy the condition expressed by the inequalities (6).Emin<E1<Emax  (6)

In addition, it is particularly preferable that the minimum electricfield strength Emin and the maximum electric field strength Emax satisfythe condition expressed by the inequalities (7).Emin<E1≦E2<Emax  (7)

In the above inequalities (6) and (7), E1 is the minimum electric fieldstrength at which the phase transition of the ferroelectric phase of oneor each of the first and second components begins, and E2 is theelectric field strength at which the phase transition is substantiallycompleted.

In addition, it is preferable to design the piezoelectric device 1according to the present embodiment so that the phase transition can berealized basically by only changing the electric field strength.Specifically, it is preferable to determine the composition of thepiezoelectric body 13 and the crystal systems between which the phasetransition is used in the piezoelectric body 13, so that the phasetransition can occur at the environmental temperature of thepiezoelectric body 13. However, when necessary, it is possible to adjustthe operational temperature of the piezoelectric device 1 so that thephase transition can occur. In any case, in order to efficiently utilizethe phase transition, it is preferable to drive the piezoelectric device1 at or near the phase transition temperature.

Since the piezoelectric device 1 according to the present invention usesthe piezoelectric body 13 formed of the perovskite oxide which isdesigned in accordance with the material design principle according tothe present invention, the piezoelectric device 1 can exhibit highpiezoelectric performance even at relatively low electric fieldstrength.

7. Inkjet Recording Apparatus

Hereinbelow, an example of an inkjet recording apparatus having theinkjet recording head 3 is explained with reference to FIGS. 7 and 8.FIG. 7 is a schematic diagram illustrating an outline of an example ofan inkjet recording apparatus having the inkjet recording head 3 of FIG.6, and FIG. 8 is a top view of a portion of the inkjet recordingapparatus of FIG. 7.

As schematically illustrated in FIG. 7, the inkjet recording apparatus100 comprises a printing unit 102, an ink reserve-and-load unit 114, asheet feeding unit 118, a decurling unit 120, a suction-type beltconveyer 122, a print detection unit 124, and a sheet output unit 126.The printing unit 102 comprises a plurality of inkjet recording heads3K, 3C, 3M, and 3Y corresponding to inks of different colors(specifically, black (K), cyan (C), magenta (M), and yellow (Y)).Hereinafter, the inkjet recording heads may be referred to as heads. Theink reserve-and-load unit 114 reserves the inks to be supplied to theheads 3K, 3C, 3M, and 3Y. The sheet feeding unit 118 feeds a recordingsheet 116. The decurling unit 120 eliminates curl of the recording sheet116. The suction-type belt conveyer 122 is arranged to face the nozzlefaces (ink-discharge faces) of the printing unit 102, and conveys therecording sheet 116 while maintaining the flatness of the recordingsheet 116. The print detection unit 124 reads an image printed on therecording sheet 116 by the printing unit 102. The sheet output unit 126externally outputs a printed recording sheet 116.

Each of the heads 3K, 3C, 3M, and 3Y constituting the printing unit 102corresponds to the inkjet recording head according to the presentembodiment as explained before, although, in order to realize a linearhead (explained later), each inkjet recording head used in the inkjetrecording apparatus 100 comprises a plurality of piezoelectric deviceson the lower electrode 12, and a plurality of ink chambers and aplurality of ink outlets are arranged in correspondence with thearrangement of the plurality of piezoelectric devices on the lowerelectrode 12.

The decurling unit 120 performs decurling of the recording sheet 116 byheating the recording sheet 116 with a heating drum 130 so as toeliminate the curl produced in the sheet feeding unit 118.

In the case where the inkjet recording apparatus 100 uses roll paper, acutter 128 for cutting the roll paper into desired size is arranged inthe stage following the decurling unit 120. The cutter 128 isconstituted by a fixed blade 128A and a round blade 128B. The fixedblade 128A has a length equal to or greater than the width of theconveying path of the recording sheet 116, and is arranged on the sideopposite to the print side of the recording sheet 116. The round blade128B is arranged opposite to the fixed blade 128A on the print side ofthe recording sheet 116, and moves along the fixed blade 128A. In theinkjet recording apparatuses using cut paper, the cutter 128 isunnecessary.

After the roll paper is decurled and cut into the recording sheet 116,the recording sheet 116 is transferred to the suction-type belt conveyer122. The suction-type belt conveyer 122 is constituted by rollers 131and 132 and an endless belt 133. The rollers 131 and 132 are placedapart and the endless belt 133 is looped around the rollers 131 and 132in such a manner that at least portions of the endless belt 133 whichface the nozzle faces of the printing unit 102 and the sensor face ofthe print detection unit 124 are flat and horizontal.

The endless belt 133 has a width greater than the width of the recordingsheet 116, and a great number of suction pores (not shown) are formedthrough the endless belt 133. A suction chamber 134 is arranged insidethe loop of the endless belt 133 at the position opposite to the nozzlefaces of the printing unit 102 and the sensor face of the printdetection unit 124, and suctioned by a fan 135, so that a negativepressure is generated in the suction chamber 134, and the recordingsheet 116 on the endless belt 133 is held by suction.

The power of a motor (not shown) is transmitted to at least one of therollers 131 and 132 so that the endless belt 133 is driven clockwise inFIG. 7, and the recording sheet 116 held on the endless belt 133 ismoved from left to right in FIG. 7.

In the case of borderless printing, ink can be deposited on the endlessbelt 133. Therefore, in order to clean the endless belt 133, a beltcleaning unit 136 is arranged at a predetermined (appropriate) positionoutside the loop of the endless belt 133 and the printing region.

A heating fan 140 is arranged on the upstream side of the printing unit102 above the conveying path of the recording sheet 116 (which isrealized by the suction-type belt conveyer 122). The heating fan 140blows heated air to the recording sheet 116 before printing so as toheat the recording sheet 116 and facilitate drying of deposited ink.

Each of the heads 3K, 3C, 3M, and 3Y in the printing unit 102 is aso-called full-line type head, which is a linear head having a lengthcorresponding to the maximum width of the recording sheet 116, and beingarranged across the width of the recording sheet 116 (i.e., in the mainscanning direction perpendicular to the feeding direction of therecording sheet 116) as illustrated in FIG. 8. Specifically, each of theheads 3K, 3C, 3M, and 3Y is a linear head in which the aforementionedplurality of ink-discharge outlets (nozzles) are arrayed over a lengthexceeding the maximum length of a side of the largest recording sheet116 on which the inkjet recording apparatus 100 can print an image. Theheads 3K, 3C, 3M, and 3Y corresponding to the inks of the differentcolors are arrayed upstream in this order along the feeding direction asillustrated in FIG. 8. Thus, a color image can be printed on therecording sheet 116 by discharging the inks of the different colorswhile conveying the recording sheet 116.

The print detection unit 124 may be constituted by, for example, a linesensor which takes an image formed of spots of the inks discharged fromthe printing unit 102, and detects, from the image taken by the linesensor, incomplete discharge, which can be caused by clogging of anozzle or the like.

A rear drying unit 142 for drying the printed surface of the recordingsheet 116 is arranged in the stage following the print detection unit124. For example, the rear drying unit 142 is realized by a heating fanor the like. Since it is preferable to avoid contact with the printedsurface before the ink on the printed surface is completely dried, it ispreferable that the rear drying unit 142 dry the ink on the printedsurface by blowing heated air.

In order to control the glossiness of the image printed on the recordingsheet 116, a heating-and-pressurizing unit 144 is arranged in the stagefollowing the rear drying unit 142. The heating-and-pressing unit 144comprises a pressure roller 145 with a surface having predeterminedprojections and depressions, and transfers the predetermined projectionsand depressions to the printed surface of the recording sheet 116 bypressing the printed surface with the pressure roller 145 while heatingthe printed surface.

Finally, the printed recording sheet 116 produced as above is outputtedfrom the sheet output unit 126. It is preferable to separately outputtest prints and prints for practical use. Therefore, the sheet outputunit 126 includes a first output unit 126A for the prints for practicaluse and a second output unit 126B for the test prints. Although notshown, the inkjet recording apparatus 100 further comprises a sortingunit which sorts the printed recording sheets 116 into the test printsand the prints for practical use, and sends the test prints to the firstoutput unit 126B, and the prints for practical use to the second outputunit 126A.

Further, in the case where both of a test image and an image forpractical use are concurrently printed on a recording sheet 116, it ispossible to arrange a cutter 148, and separate a first portion of therecording sheet 116 on which the test image is printed and a secondportion of the recording sheet 116 on which the image for practical useis printed.

8. Concrete Examples of the Present Invention

The present inventor has produced concrete examples 1 to 3 of thepiezoelectric device according to the present invention and comparisonexamples 1 and 2 of the conventional piezoelectric devices as indicatedbelow.

8.1 Concrete Example 1

The concrete example 1 of the piezoelectric device according to thepresent invention is produced as follows.

First, an SiO₂/Si substrate in which the SiO₂ layer has a thickness of0.1 micrometers is prepared. Then, a contact layer of titanium having athickness of 20 nm is formed on the SiO₂/Si substrate, and a lowerelectrode of platinum having a thickness of 0.2 micrometers is formed bysputtering. Thereafter, a piezoelectric film of Nb-doped PZT, Pb(Ti, Zr,Nb)O₃ (specifically, PbZr_(0.44)Ti_(0.44)Nb_(0.12)O₃) having a thicknessof 5.0 micrometers is formed by sputtering at the substrate temperatureof 525° C. Further, an upper electrode of platinum having a thickness of0.2 micrometers is formed on the piezoelectric film by sputtering. Thus,a piezoelectric device according to the present invention is obtained.

8.2 Comparison Example 1

The comparison example 1 of the piezoelectric device is produced asfollows.

The piezoelectric device as the comparison example 1 is different fromthe concrete example 1 only in that the piezoelectric film is made ofundoped PZT (specifically, PbZr_(0.44)Ti_(0.48)O₃).

8.3 Comparison of Concrete Example 1 and Comparison Example 1

The present inventor has performed high-resolution X-ray diffraction(high-resolution XRD) measurement of the piezoelectric film in each ofthe concrete example 1 and the comparison example 1, and separated peaksin the obtained XRD profiles. FIGS. 9A and 9B respectively showhigh-resolution XRD profiles of the piezoelectric films in the concreteexample 1 and the comparison example 1.

As indicated in FIG. 9B, only a strong (200) diffraction peak of arhombohedral phase (R) and a weak (200) diffraction peak and a weak(002) diffraction peak of a tetragonal phase (T) are detected in thehigh-resolution XRD profile of the piezoelectric film in the comparisonexample 1 (using undoped PZT). That is, it is confirmed that thepiezoelectric film in the comparison example 1 has a two-phasemixed-crystal structure composed of the tetragonal phase and thetetragonal phase.

On the other hand, as indicated in FIG. 9A, a (200) diffraction peak ofa rhombohedral phase (R), a (200) diffraction peak and a (002)diffraction peak of a tetragonal phase (T), and a diffraction peak of athird phase different from the rhombohedral and tetragonal phases aredetected in the high-resolution XRD profile of the piezoelectric film inthe concrete example 1 (using Nb-doped PZT).

The present inventor considers that the diffraction peak of the thirdphase corresponds to domains of PbNbO₃, which are produced by the dopingwith Nb, and the present inventor has obtained the lattice constant ofthe domains as 4.08 angstroms on the basis of the diffraction peak ofthe third phase. The obtained value, 4.08 angstroms, of the latticeconstant approximately coincides with the value of the lattice constantwhich is expected when PbNbO₃, is assumed to form the cubic orpseudocubic phase.

Next, the present inventor has performed structural analysis of thepiezoelectric film in each of the concrete example 1 and the comparisonexample 1 by EXAFS (Extended X-ray Absorption Fine Structure)measurement. FIG. 10A shows a Pb L3-edge EXAFS spectrum of thepiezoelectric film of undoped PZT in the comparison example 1, FIG. 10Bshows a Ti K-edge EXAFS spectrum of the piezoelectric film of undopedPZT in the comparison example 1, FIG. 10C shows a Zr K-edge EXAFSspectrum of the piezoelectric film of undoped PZT in the comparisonexample 1, and FIG. 10D shows a Nb K-edge EXAFS spectrum of thepiezoelectric film of Nb-doped PZT in the concrete example 1. Then, thebond distances in the bonds Ti—O, Zr—O, and Nb—O are obtained asindicated in Table 2.

As indicated in Table 2, the first bond distance near 1.8 angstroms andthe second bond distance near 2.0 angstroms are detected for the bondsTi—O in the piezoelectric (undoped PZT) film in each of the comparisonexample 1 and the concrete example 1. This means that the Ti ions resideat two different distances apart from the centers of unit cells (asillustrated in FIG. 4A), and the crystal system of PT (PbTiO₃) is therhombohedral or tetragonal system.

On the other hand, only one bond distance is detected for the bonds Nb—Oin the piezoelectric (Nb-doped PZT) film in the concrete example 1. Thismeans that the Nb ions reside at the centers of unit cells. Althoughgenerally there is a possibility that the Nb ions reside at the centersof unit cells of rhombohedral or tetragonal crystals when the latticesof the rhombohedral or tetragonal crystals are distorted, the presentinventor conjectures that the crystal system of PbNbO₃ is the cubic orpseudocubic system, in consideration of the facts that the tolerancefactor (TF) of is PbNbO₃ is near 1.0, and the lattice constant of thethird phase which is obtained from the high-resolution XRD profileapproximately coincides with the value of the lattice constant which isexpected when PbNbO₃ is assumed to form the cubic or pseudocubic phase.

Based on the measurement results of the high-resolution XRD and theEXAFS, it is possible to consider that the piezoelectric film formed inthe concrete example 1 has a three-phase mixed-crystal structure of thetetragonal phase, the rhombohedral phase, and the cubic or pseudocubicphase.

TABLE 2 Band Distance (Å) Ti—O Ti—O Zr—O Nb—O Concrete Nb-PZT 1.82 1.992.03 1.96 Example 1 Comparison PZT 1.83 2.01 2.04 — 1

8.4 Concrete Example 2

The concrete example 2 of the piezoelectric device according to thepresent invention is produced as follows.

First, an SiO₂/Si substrate in which the SiO₂ layer has a thickness of0.1 micrometers is prepared. Then, a contact layer of titanium having athickness of 20 nm is formed on the SiO₂/Si substrate, and a lowerelectrode of platinum having a thickness of 0.2 micrometers is formed bysputtering. Thereafter, a piezoelectric film of Nb-doped PZT, Pb(Ti, Zr,Nb)O₃ (specifically, PbZr_(0.44)Ti_(0.44)Nb_(0.12)O₃) having a thicknessof 5.0 micrometers is formed by sputtering at the substrate temperatureof 525° C., and is then annealed at 650° C. in oxygen atmosphere.Further, an upper electrode of platinum having a thickness of 0.2micrometers is formed on the piezoelectric film by sputtering. Thus, apiezoelectric device according to the present invention is obtained.

The present inventor has performed high-resolution X-ray diffraction(high-resolution XRD) measurement of the above piezoelectric film in theconcrete example 2, and confirmed that the piezoelectric film has athree-phase mixed-crystal structure similar to the concrete example 1.Further, the present inventor has performed XRD measurement whileapplying an electric field, and confirmed that a portion of arhombohedral phase (R) transitions to a tetragonal phase when theelectric field is applied along the <001> direction. FIG. 11 showshigh-resolution XRD profiles of the piezoelectric body in the concreteexample 2 which are obtained at different levels of the electric fieldstrength. FIG. 11 indicates that the diffraction peak of therhombohedral phase (R) shifts when the electric field strength isincreased. This is because when the electric field strength isincreased, the crystal lattice of the rhombohedral phase expands in thedirection of the applied electric field, so that the lattice constantincreases. This is a piezoelectric distortion caused by theengineered-domain effect. FIG. 11 also indicates that the peakintensities of the (200) and (002) diffraction peaks of the tetragonalphase (T) increase when the electric field strength increases. This isbecause when the electric field strength is increased, a portion of therhombohedral phase (R) transitions to the tetragonal phase.

Further, the present inventor has measured the piezoelectric coefficientd₃₁ in the range of the electric field strength between the minimumelectric field strength Emin (=0 kV/cm (<E1)) and the maximum electricfield strength Emax (=100 kV/cm (>E2)) by using a cantilever, andobtained the value of 250 pm/V. This value of the piezoelectriccoefficient d₃₁ is the highest value in the world at present as long asthe present inventor knows.

8.5 Comparison Example 2

The comparison example 2 of the piezoelectric device is produced asfollows.

First, a lower electrode of platinum having a thickness of 0.2micrometers is formed by sputtering on a (100) MgO substrate. Then, apiezoelectric film of undoped PZT, Pb(Ti, Zr)O₃ (specifically,PbZr_(0.55)Ti_(0.45)O₃) having a thickness of 5 micrometers is formed bypulsed-laser deposition at the substrate temperature of 525° C. Further,an upper electrode of platinum having a thickness of 0.2 micrometers isformed on the piezoelectric film by sputtering. Thus, a piezoelectricfilm as the comparison example 2 is obtained.

The present inventor has performed high-resolution X-ray diffraction(high-resolution XRD) measurement of the above piezoelectric film in thecomparison example 2, and confirmed that the piezoelectric film isformed of a rhombohedral phase (R) having crystal orientation (with thedegree of crystal orientation of 95%) along the <001> direction when noelectric field is applied, and the rhombohedral phase (R) transitions toa tetragonal phase (T) when an electric field is applied in the <001>direction. In this example, the direction of the applied electric fieldis identical to the orientation of the spontaneous polarization axisafter the phase transition. The minimum electric field strength E1 atwhich the phase transition begins and the electric field strength E2 atwhich the phase transition from the rhombohedral phase (R) to thetetragonal phase (T) is substantially completed are respectively 110kV/cm and 160 kV/cm.

Further, the present inventor has measured the piezoelectric coefficientd₃₁ of the piezoelectric film in the range of the electric fieldstrength between the minimum electric field strength Emin (=50 kV/cm(<E1)) and the maximum electric field strength Emax (=200 kV/cm (>E2))by using a cantilever, and obtained the value of 190 pm/V.

8.6 Comparison of Concrete Example 2 and Comparison Example 2

In each of the concrete example 2 and the comparison example 2, phasetransition of a ferroelectric phase occurs in the piezoelectric film,and the electric field is applied to the piezoelectric film in thedirection identical to the orientation of the spontaneous polarizationaxis of a ferroelectric phase after the phase transition under thecondition that Emin<E1≦E2<Emax. Although the concrete example 2 and thecomparison example 2 are similar in the above respects, thepiezoelectric film in the concrete example 2 formed of Nb-doped PZTexhibits greater distortion at lower electric field strength than thepiezoelectric film in the comparison example 2 formed of undoped PZT.That is, the effectiveness of the material design principle according tothe present invention is proved. The present inventor considers that thepiezoelectric film in the concrete example 2 exhibits the greatdistortion at the relatively low electric field strength for the reasonwhich is explained before with reference to FIGS. 3A, 3B, and 3C byusing the phase transition model 1.

8.7 Concrete Example 3

The concrete example 3 of the piezoelectric device according to thepresent invention is produced as follows.

First, an SiO₂/Si substrate in which the SiO₂ layer has a thickness of0.1 micrometers is prepared. Then, a contact layer of titanium having athickness of 20 nm is formed on the SiO₂/Si substrate, and a lowerelectrode of platinum having a thickness of 0.13 micrometers is formedby sputtering. Thereafter, a piezoelectric film of Nb-doped PZT, Pb(Ti,Zr, Nb)O₃ having a thickness of 2.4 micrometers is formed by sputteringat the substrate temperature of 525° C. Further, an upper electrode ofplatinum having a thickness of 0.2 micrometers is formed on thepiezoelectric film by sputtering. Thus, a piezoelectric device accordingto the present invention is obtained.

The present inventor has performed thickness measurement and X-rayfluorescence (XRF) measurement of the piezoelectric film as the concreteexample 3. Table 3 shows the measured values of the thickness and thecomposition (the mole fraction of each constituent element and the molarratio of Zr/(Zr+Ti)).

TABLE 3 Film Thickness Pb Zr Ti Nb Pr Ec (μm) Zr + Ti + Nb = 1 Zr/(Zr +Ti) ε (μC/cm²) (kV/cm) d₃₁ Concrete 2.4 1.05 0.47 0.44 0.09 0.52 61024.6 69.1 250 Example 3

8.8 Evaluation of Concrete Example 3

The present inventor has performed high-resolution X-ray diffraction(high-resolution XRD) measurement of the piezoelectric film in theconcrete example 3, and separated peaks in the obtained XRD profiles.FIG. 12 shows a high-resolution XRD profile and the separateddiffraction peaks of the piezoelectric film in the concrete example 3.In FIG. 12, the diffraction angle (in degrees) of each diffraction peak,the lattice constant obtained on the basis of the diffraction angle, andthe integrated intensity Int (%) of each diffraction peak normalized bythe integrated intensity of the largest diffraction peak are indicatedin association with each other.

As indicated in FIG. 12, only a strong (200) diffraction peak of arhombohedral phase (R) and a weak diffraction peak (which appears on theleft side of the strong (200) diffraction peak of the rhombohedral phase(R)) are detected in the high-resolution XRD profile of thepiezoelectric film in the comparison example 3. Normally, XRD profileshaving a (200) diffraction peak of a rhombohedral phase (R) and a (002)diffraction peak of a tetragonal phase (T) also have a (200) diffractionpeak of the tetragonal phase (T) on the right side of the (200)diffraction peak of the rhombohedral phase (R). Therefore, the XRDprofile of FIG. 12 is different from the normal XRD profiles as above.Therefore, the present inventor considers that the weak diffraction peakis a (002) diffraction peak of a tetragonal phase (T), which correspondsto the tetragonal domains behaving as the seeds of phase transitionexplained before with reference to FIGS. 5A, 5B, and 5C by using thephase transition model 2.

The present inventor has performed XRD measurement of the piezoelectricfilm in the concrete example 3 while applying an electric field to thepiezoelectric film in the <001> direction, and confirmed that therhombohedral phase (R) transitions to the tetragonal phase (T).

Furthermore, the present inventor has produced a diaphragm-typepiezoelectric device as the concrete example 3, measured a relationshipbetween the electric field strength and the displacement in thepiezoelectric device and a relationship between the electric fieldstrength and the dielectric polarization in the piezoelectric device,and obtained curves indicating the relationships. In the curveindicating the relationship between the electric field strength and thedisplacement in the piezoelectric device (the electricfield-displacement curve), an inflection point (at which the gradient ofthe electric field-displacement curve changes) exists at each of theminimum electric field strength E1 (at which the phase transitionbegins) and the electric field strength E2 (at which the phasetransition ends). The minimum electric field strength E1 at which thephase transition from the rhombohedral phase (R) to the tetragonal phase(T) begins is detected to be 45 kV/cm, and the electric field strengthE2 at which the phase transition from the rhombohedral phase (R) to thetetragonal phase (T) is substantially completed is detected to be 67kV/cm. These results of the measurement on the basis of the electricfield-displacement curve coincide with the results of the XRDmeasurement of the piezoelectric film to which the electric field isapplied. Thus, the above results obtained from the electricfield-displacement curve and the XRD measurement confirm that the phasetransition occurs.

Further, the present inventor has measured the piezoelectric coefficientd₃₁ of the piezoelectric film in the range of the electric fieldstrength between the minimum electric field strength Emin (=0 kV/cm(<E1)) and the maximum electric field strength Emax (=100 kV/cm (>E2)),and obtained the value of 250 pm/V. Table 3 also shows the values of thepiezoelectric coefficient d₃₁ (obtained by driving the piezoelectricdevice as the concrete example 3 in the range of the electric fieldstrength from 0 to 100 kV/cm) and the dielectric constant ∈ measured atthe frequency of 100 kHz.

Moreover, the present inventor has obtained the curve indicating therelationship between the electric field strength and the dielectricpolarization in the piezoelectric device as the concrete example 3 bydriving the piezoelectric device in the range of the electric fieldstrength from −200 to 200 kV/cm), and measured the values of theresidual dielectric polarization Pr (which is measured at the electricfield strength equal to zero) and the coercive electric field Ec, whichare also indicated in Table 3.

8.9 Concrete Example 4

The concrete example 4 according to the present invention is produced asfollows.

The present inventor has designed a material for a film of theperovskite oxide Bi (Al,Fe,Sc)O₃ is designed by selecting BiAlO₃ (havingthe tolerance factor TF of 1.012) as the first component (having thetolerance factor TF greater than 1.0), BiScO₃ (having the tolerancefactor TF of 0.911) as the second component (having the tolerance factorTF smaller than 1.0), and BiFeO₃ (having the tolerance factor TF of0.960) as the third component, and determined the composition BiAl_(0.6)Fe_(0.35)Sc_(0.05)O₃ (which is at or near the MPB) on the basisof the recognition that the tolerance factor TF of BiAl_(0.6)Fe_(0.35)Sc_(0.05)O₃ is 0.989.

Next, the film of the perovskite oxide having the above composition isproduced as follows.

First, an SiO₂/Si substrate in which the SiO₂ layer has a thickness of0.1 micrometers is prepared. Then, a contact layer of titanium having athickness of 20 nm is formed on the SiO2₂/Si substrate, and a lowerelectrode of platinum having a thickness of 0.2 micrometers is formed bysputtering. Thereafter, a piezoelectric film of Bi (Al,Fe,Sc)O₃(specifically, Bi Al_(0.6)Fe_(0.35)Sc_(0.05)O₃) having a thickness of0.6 micrometers is formed by PLD (pulsed-laser deposition) at thesubstrate temperature of 670° C.

The present inventor has performed X-ray diffraction (XRD) measurementof the piezoelectric film formed as above, and the XRD profile of thepiezoelectric body as the concrete example 4 is shown in FIG. 13A. Asindicated in FIG. 13A, the above film is formed of a single perovskitephase, and is preferentially (100)/(001) oriented. A magnification of aportion of the XRD profile of FIG. 13A around the diffraction angle (2θ)of 46 degrees is shown in FIG. 13B. FIG. 13B shows that the (200)diffraction peak of the above film has a left shoulder, and isasymmetric. As indicated in FIG. 13B, the present inventor has confirmedthat the tetragonal phase (T) and the rhombohedral phase (R) are mixedin the above film.

8.10 Comparison Example 3

ABi (Al,Fe,Sc) O₃ film as the comparison example 3 is produced in asimilar manner to the concrete example 4 except that the piezoelectricfilm as the comparison example 3 is made ofBiAl_(0.3)Fe_(0.65)Sc_(0.05)O₃, the tolerance factor TF of which is0.973. The present inventor has performed X-ray diffraction (XRD)measurement of the piezoelectric film as the comparison example 3, andconfirmed that the piezoelectric film as the comparison example 3 isformed of a single perovskite phase, and only the rhombohedral phase (R)is observed.

9. Additional Matters

The piezoelectric devices according to the present invention can bepreferably used in piezoelectric actuators, ferroelectric memories(FRAMs), and the like, where the piezoelectric actuators may be mountedin the inkjet recording heads, the magnetic recording-and-reproductionheads, MEMS (micro electromechanical systems) devices, micropumps,ultrasonic probes, and the like.

1. A process for producing a piezoelectric oxide having a compositionexpressed by a compositional formula:(A, B, C)(D, E, F)O₃, where each of A, B, C, D, E, and F represents oneor more metal elements, A, B, and C represent Pb, D, E, and F representTi, Zr, and M, respectively, M is at least one metal element selectedfrom the group consisting of Sn, Nb, Ta, Mo, W, Ir, Os, Pd, Pt, Re, Mn,Co, Ni, V, and Fe, and O represents the oxygen atom, wherein a ratio ofeach of a total molar amount of A, B and C and a total molar amount ofD, E and F to a molar amount of oxygen atoms may deviate from 1:3 withina range in which the composition expressed by the compositional formula(A, B, C)(D, F, F)O₃ can form a perovskite structure; and wherein thecomposition of said perovskite oxide satisfies conditions (1), (2), (3),and (4):0.98≦TF(P)≦1.01,  (1)TF(ADO₃)>1.0,  (2)TF(BEO₃)<1.0, and  (3)TF(BEO₃)<TF(CFO₃)<TF(ADO₃),  (4) where TF(P) is a tolerance factor ofthe piezoelectric oxide expressed by the compositional formula (A, B,C)(D, E, F)O₃, TF(ADO₃) is a tolerance factor of a compound expressed bya compositional formula ADO₃, TF(BEO₃) is a tolerance factor of acompound expressed by a compositional formula BEO₃, and TF(CFO₃) is atolerance factor of a compound expressed by a compositional formulaCFO₃.
 2. A process according to claim 1, wherein said composition of theperovskite oxide is determined so as to further satisfy condition (5):0.98≦TF(CFO₃)≦1.02  (5).